feat: Google Tasks bidirectional sync and CLI refactor (#59)

* feat(gtasks): add Google Tasks bidirectional sync

Problem: pending.nvim only supported one-way push to Google Calendar.
Users who use Google Tasks had no way to sync tasks bidirectionally.

Solution: add `lua/pending/sync/gtasks.lua` backend with OAuth PKCE
auth, push/pull/sync actions, and field mapping between pending tasks
and Google Tasks (category↔tasklist, `priority`/`recur` via notes).

* refactor(cli): promote sync backends to top-level subcommands

Problem: `:Pending sync gtasks auth` required an extra `sync` keyword
that added no value and made the command unnecessarily verbose.

Solution: route `gtasks` and `gcal` as top-level `:Pending` subcommands
via `SYNC_BACKEND_SET` lookup. Tab completion introspects backend
modules for available actions instead of hardcoding `{ 'auth', 'sync' }`.

* docs(gtasks): document Google Tasks backend and CLI changes

Problem: vimdoc had no coverage for the gtasks backend and still
referenced the old `:Pending sync <backend>` command form.

Solution: add `:Pending-gtasks` and `:Pending-gcal` command sections
with per-action docs, update sync backend interface, and add gtasks
config example.

* ci: format

doc/pending.txt

@@ -41,6 +41,7 @@ Features: ~
 - Foldable category sections (`zc`/`zo`) in category view
 - Omnifunc completion for `cat:`, `due:`, and `rec:` tokens (`<C-x><C-o>`)
 - Google Calendar one-way push via OAuth PKCE
+- Google Tasks bidirectional sync via OAuth PKCE
 
 ==============================================================================
 CONTENTS                                                   *pending-contents*
@@ -63,15 +64,16 @@ CONTENTS                                                   *pending-contents*
  16. Recipes ............................................... |pending-recipes|
  17. Sync Backends ................................... |pending-sync-backend|
  18. Google Calendar .......................................... |pending-gcal|
- 19. Data Format .............................................. |pending-data|
- 20. Health Check ........................................... |pending-health|
+ 19. Google Tasks ............................................ |pending-gtasks|
+ 20. Data Format .............................................. |pending-data|
+ 21. Health Check ........................................... |pending-health|
 
 ==============================================================================
 REQUIREMENTS                                           *pending-requirements*
 
 - Neovim 0.10+
 - No external dependencies for local use
-- `curl` and `openssl` are required for the `gcal` sync backend
+- `curl` and `openssl` are required for the `gcal` and `gtasks` sync backends
 
 ==============================================================================
 INSTALL                                                      *pending-install*
@@ -146,24 +148,42 @@ COMMANDS                                                    *pending-commands*
     Populate the quickfix list with all tasks that are overdue or due today.
     Open the list with |:copen| to navigate to each task's category.
 
-                                                               *:Pending-sync*
-:Pending sync {backend} [{action}]
-    Run a sync action against a named backend. {backend} is required — bare
-    `:Pending sync` prints a usage message. {action} defaults to `sync`
-    when omitted. Each backend lives at `lua/pending/sync/<name>.lua`.
+                                                              *:Pending-gtasks*
+:Pending gtasks [{action}]
+    Run a Google Tasks sync action. {action} defaults to `sync` when omitted.
+
+    Actions: ~
+        `sync`    Push local changes then pull remote changes (default).
+        `push`    Push local changes to Google Tasks only.
+        `pull`    Pull remote changes from Google Tasks only.
+        `auth`    Run the OAuth authorization flow.
 
     Examples: >vim
-        :Pending sync gcal          " runs gcal.sync()
-        :Pending sync gcal auth     " runs gcal.auth()
-        :Pending sync gcal sync     " explicit sync (same as bare)
+        :Pending gtasks             " push then pull (default)
+        :Pending gtasks push        " push local → Google Tasks
+        :Pending gtasks pull        " pull Google Tasks → local
+        :Pending gtasks auth        " authorize
 <
 
-    Tab completion after `:Pending sync ` lists discovered backends.
-    Tab completion after `:Pending sync gcal ` lists available actions.
+    Tab completion after `:Pending gtasks ` lists available actions.
+    See |pending-gtasks| for full details.
 
-    Built-in backends: ~
+                                                               *:Pending-gcal*
+:Pending gcal [{action}]
+    Run a Google Calendar sync action. {action} defaults to `sync` when
+    omitted.
 
-    `gcal`    Google Calendar one-way push. See |pending-gcal|.
+    Actions: ~
+        `sync`    Push tasks with due dates to Google Calendar (default).
+        `auth`    Run the OAuth authorization flow.
+
+    Examples: >vim
+        :Pending gcal               " push to Google Calendar (default)
+        :Pending gcal auth          " authorize
+<
+
+    Tab completion after `:Pending gcal ` lists available actions.
+    See |pending-gcal| for full details.
 
                                                             *:Pending-filter*
 :Pending filter {predicates}
@@ -590,6 +610,9 @@ loads: >lua
           calendar = 'Pendings',
           credentials_path = '/path/to/client_secret.json',
         },
+        gtasks = {
+          credentials_path = '/path/to/client_secret.json',
+        },
       },
     }
 <
@@ -870,21 +893,30 @@ Open tasks in a new tab on startup: >lua
 SYNC BACKENDS                                           *pending-sync-backend*
 
 Sync backends are Lua modules under `lua/pending/sync/<name>.lua`. Each
-module returns a table conforming to the backend interface: >lua
+backend is exposed as a top-level `:Pending` subcommand: >vim
+    :Pending gtasks [action]
+    :Pending gcal [action]
+<
+
+Each module returns a table conforming to the backend interface: >lua
 
     ---@class pending.SyncBackend
     ---@field name string
     ---@field auth fun(): nil
     ---@field sync fun(): nil
+    ---@field push? fun(): nil
+    ---@field pull? fun(): nil
     ---@field health? fun(): nil
 <
 
 Required fields: ~
     {name}    Backend identifier (matches the filename).
-    {sync}    Main sync action. Called by `:Pending sync <name>`.
-    {auth}    Authorization flow. Called by `:Pending sync <name> auth`.
+    {sync}    Main sync action. Called by `:Pending <name>`.
+    {auth}    Authorization flow. Called by `:Pending <name> auth`.
 
 Optional fields: ~
+    {push}    Push-only action. Called by `:Pending <name> push`.
+    {pull}    Pull-only action. Called by `:Pending <name> pull`.
     {health}  Called by `:checkhealth pending` to report backend-specific
               diagnostics (e.g. checking for external tools).
 
@@ -923,7 +955,7 @@ Fields: ~
                          that Google provides or as a bare credentials object.
 
 OAuth flow: ~
-On the first `:Pending sync gcal` call the plugin detects that no refresh token
+On the first `:Pending gcal` call the plugin detects that no refresh token
 exists and opens the Google authorization URL in the browser using
 |vim.ui.open()|. A temporary local HTTP server listens on port 18392 for the
 OAuth redirect. The PKCE (Proof Key for Code Exchange) flow is used —
@@ -933,7 +965,7 @@ authorization code is exchanged for tokens and the refresh token is stored at
 use the stored refresh token and refresh the access token automatically when
 it is about to expire.
 
-`:Pending sync gcal` behavior: ~
+`:Pending gcal` behavior: ~
 For each task in the store:
 - A pending task with a due date and no existing event: a new all-day event is
   created and the event ID is stored in the task's `_extra` table.
@@ -946,6 +978,67 @@ For each task in the store:
 A summary notification is shown after sync: `created: N, updated: N,
 deleted: N`.
 
+==============================================================================
+GOOGLE TASKS                                                  *pending-gtasks*
+
+pending.nvim can sync tasks bidirectionally with Google Tasks. Each
+pending.nvim category maps to a Google Tasks list of the same name. Lists are
+created automatically on first sync.
+
+Configuration: >lua
+    vim.g.pending = {
+      sync = {
+        gtasks = {
+          credentials_path = '/path/to/client_secret.json',
+        },
+      },
+    }
+<
+
+                                                        *pending.GtasksConfig*
+Fields: ~
+    {credentials_path}   (string)
+                         Path to the OAuth client secret JSON file downloaded
+                         from the Google Cloud Console. Default:
+                         `stdpath('data')..'/pending/gtasks_credentials.json'`.
+                         Accepts the `installed` wrapper format or a bare
+                         credentials object.
+
+OAuth flow: ~
+Same PKCE flow as the gcal backend; listens on port 18393. Tokens are stored
+at `stdpath('data')/pending/gtasks_tokens.json`. Run `:Pending gtasks auth`
+to authorize; subsequent syncs refresh the token automatically.
+
+`:Pending gtasks` actions: ~
+
+`:Pending gtasks` (or `:Pending gtasks sync`) runs push then pull. Use
+`:Pending gtasks push` or `:Pending gtasks pull` to run only one direction.
+
+Push (local → Google Tasks, `:Pending gtasks push`):
+- Pending task with no `_gtasks_task_id`: created in the matching list.
+- Pending task with an existing ID: updated in Google Tasks.
+- Done task with an existing ID: marked `completed` in Google Tasks.
+- Deleted task with an existing ID: deleted from Google Tasks.
+
+Pull (Google Tasks → local, `:Pending gtasks pull`):
+- GTasks task already known (matched by `_gtasks_task_id`): updated locally
+  if `gtasks.updated` timestamp is newer than `task.modified`.
+- GTasks task not known locally: created as a new pending.nvim task in the
+  category matching the list name.
+
+Field mapping: ~
+    {title}     ↔ task description
+    {status}    `needsAction` ↔ `pending`, `completed` ↔ `done`
+    {due}       date-only; time component ignored (GTasks limitation)
+    {notes}     serializes extra fields: `pri:1 rec:weekly`
+
+The `notes` field is used exclusively for pending.nvim metadata. Any existing
+notes on tasks created outside pending.nvim are parsed for known tokens and
+the remainder is ignored.
+
+Recurrence (`rec:`) is stored in notes for round-tripping but is not
+expanded by Google Tasks (GTasks has no recurrence API).
+
 ==============================================================================
 DATA FORMAT                                                    *pending-data*
 
@@ -979,7 +1072,8 @@ Task fields: ~
 
 Any field not in the list above is preserved in `_extra` and written back on
 save. This is used internally to store the Google Calendar event ID
-(`_gcal_event_id`) and allows third-party tooling to annotate tasks without
+(`_gcal_event_id`) and Google Tasks IDs (`_gtasks_task_id`,
+`_gtasks_list_id`), and allows third-party tooling to annotate tasks without
 data loss.
 
 The `version` field is checked on load. If the file version is newer than the