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

lua/pending/config.lua

@@ -10,8 +10,12 @@
 ---@field calendar? string
 ---@field credentials_path? string
 
+---@class pending.GtasksConfig
+---@field credentials_path? string
+
 ---@class pending.SyncConfig
 ---@field gcal? pending.GcalConfig
+---@field gtasks? pending.GtasksConfig
 
 ---@class pending.Keymaps
 ---@field close? string|false