Google Tasks sync + shared OAuth module (#60)

* 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

* refactor(sync): extract shared OAuth into `oauth.lua`

Problem: `gcal.lua` and `gtasks.lua` duplicated ~250 lines of identical
OAuth code (token management, PKCE flow, credential loading, curl
helpers, url encoding).

Solution: Extract a shared `OAuthClient` metatable in `oauth.lua` with
module-level utilities and instance methods. Both backends now delegate
all OAuth to `oauth.new()`. Skip `oauth` in `health.lua` backend
discovery by checking for a `name` field.

* feat(sync): ship bundled OAuth credentials

Problem: Users must manually create a Google Cloud project and place a
credentials JSON file before sync works — terrible onboarding.

Solution: Add `client_id`/`client_secret` fields to `GcalConfig` and
`GtasksConfig`. `oauth.lua` resolves credentials in three tiers: config
fields, credentials file, then bundled defaults (placeholders for now).

* docs(sync): document bundled credentials and config fields

* ci: format

lua/pending/config.lua

@@ -9,9 +9,13 @@
 ---@class pending.GcalConfig
 ---@field calendar? string
 ---@field credentials_path? string
+---@field client_id? string
+---@field client_secret? string
 
 ---@class pending.GtasksConfig
 ---@field credentials_path? string
+---@field client_id? string
+---@field client_secret? string
 
 ---@class pending.SyncConfig
 ---@field gcal? pending.GcalConfig