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`.

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',