Problem: `push_pass` updated all remote-linked tasks unconditionally,
causing unnecessary API calls and potential clobbering of remote edits
made between syncs. `pull`/`sync` never noticed when a task disappeared
from remote. `update_event` omitted `transparency` that `create_event`
set. Failure counts were absent from sync log summaries.
Solution: Introduce `_gtasks_synced_at` in `_extra` — stamped after
every successful push/pull create or update — so `push_pass` skips
tasks unchanged since last sync. Add `detect_remote_deletions` to
unlink local tasks whose remote entry disappeared from a successfully
fetched list. Surface failures as `!N` in all sync logs and
`unlinked: N` for pull/sync. Add `transparency = 'transparent'` to
`update_event`. Cover new behaviour with 7 tests in `gtasks_spec.lua`.
* 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
