* 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
