* 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
ad59e894c7