* fix(gtasks): prevent concurrent push/pull from racing on the store
Problem: `push` and `pull` both run via `oauth.async`, so issuing them
back-to-back starts two coroutines that interleave at every curl yield.
Both snapshot `build_id_index` before either has mutated the store,
which can cause push to create a remote task that pull would have
recognized as already linked, producing duplicates on Google.
Solution: guard `with_token` with a module-level `_in_flight` flag set
before `oauth.async` is called so no second operation can start during
a token-refresh yield. A `pcall` around the callback guarantees the
flag is always cleared, even on an unexpected error.
* refactor(sync): centralize `with_token` in oauth.lua with shared lock
Problem: `with_token` was duplicated in `gcal.lua` and `gtasks.lua`,
with the concurrency lock added only to the gtasks copy. Any new
backend would silently inherit the same race, and gcal back-to-back
push could still create duplicate remote calendar events.
Solution: lift `with_token` into `oauth.lua` as
`M.with_token(client, name, callback)` behind a module-level
`_sync_in_flight` guard. All backends share one implementation; the
lock covers gcal, gtasks, and any future backend automatically.
* ci: format
abf3e52277