feat(sync): diff metadata preservation, auth unification, and sync quality improvements (#74)

* feat(sync): unify Google auth under :Pending auth

Problem: users had to run `:Pending gtasks auth` and `:Pending gcal
auth` separately, producing two token files and two browser consents
for the same Google account.

Solution: introduce `oauth.google_client` with combined tasks +
calendar scopes and a single `google_tokens.json`. Remove per-backend
`auth`/`setup` from `gcal` and `gtasks`; add top-level `:Pending auth`
that prompts with `vim.ui.select` and delegates to the shared client's
`setup()` or `auth()` based on credential availability.

* docs: update vimdoc for unified Google auth

Problem: `doc/pending.txt` still documented per-backend `:Pending gtasks
auth` / `:Pending gcal auth` commands and separate token files, which no
longer exist after the auth unification.

Solution: add `:Pending auth` entry to COMMANDS and a new
`*pending-google-auth*` section covering the shared PKCE flow, combined
scopes, and `google_tokens.json`. Remove `auth` from gcal/gtasks action
tables and update all cross-references to use `:Pending auth`.

* ci: format

* feat(sync): selective push, remote deletion detection, and gcal fix

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`.

* ci: formt

lua/pending/sync/gcal.lua

@@ -97,6 +97,7 @@ local function update_event(access_token, calendar_id, event_id, task)
     summary = task.description,
     start = { date = task.due },
     ['end'] = { date = next_day(task.due or '') },
+    transparency = 'transparent',
   }
   local _, err = oauth.curl_request(
     'PATCH',