contacts: support --birthday and --notes in contacts update

[rosssivertsen]

This PR extends gog contacts update to support updating two additional People API fields that users commonly need but that weren't previously exposed:

• --birthday YYYY-MM-DD (writes People API birthdays, primary)
• --notes <text> (writes People API biographies, primary; TEXT_PLAIN)

Both flags support clearing by passing an empty string.

Implementation details:

• Fetches existing contact with birthdays,biographies included in the PersonFields mask when running update.
• Adds the appropriate UpdatePersonFields entries (birthdays, biographies).
• Parses birthday via Go time parsing (YYYY-MM-DD).

Motivation: we hit this gap while automating contact hygiene, and the underlying People API supports these fields; it's mainly a CLI UX/coverage gap.

Testing:

• go test ./... passes in a clean env.

[steipete]

Landed via temp rebase onto main.

• Gate: \pnpm gate skipped (no package.json)
  0 issues.
  ok github.com/steipete/gogcli/cmd/gog (cached)
  ? github.com/steipete/gogcli/internal/authclient [no test files]
  ok github.com/steipete/gogcli/internal/cmd 15.915s
  ok github.com/steipete/gogcli/internal/config (cached)
  ok github.com/steipete/gogcli/internal/errfmt (cached)
  ok github.com/steipete/gogcli/internal/googleapi (cached)
  ok github.com/steipete/gogcli/internal/googleauth (cached)
  ok github.com/steipete/gogcli/internal/input (cached)
  ok github.com/steipete/gogcli/internal/outfmt (cached)
  ok github.com/steipete/gogcli/internal/secrets (cached)
  ok github.com/steipete/gogcli/internal/timeparse (cached)
  ok github.com/steipete/gogcli/internal/tracking (cached)
  ok github.com/steipete/gogcli/internal/ui (cached)
  ok github.com/steipete/gogcli/scripts (cached)
• Land commit: b1a26d3
• Merge commit: 077f9a3

Thanks @rosssivertsen!