Step AAB fix: fold admin surface into existing /dev, not a parallel page

Reverts the standalone /admin page from the previous commit — /dev was
already the canonical ops surface (bootstrap training, datasets,
fine-tune jobs, debug logs, token-gated). Shipping /admin alongside
created two admin consoles.

- Deleted app/admin/page.js and the 'admin' link in the main chat
  header.
- Kept the three /api/admin/* action endpoints (bench, seed-blind,
  recompute-skiplist) — they're real endpoints with no UI coupling.
- Extended /dev with a third tab 'Deploy & validate' that renders
  the same cards the deleted /admin had: deploy health dots, run
  bench, seed blind-eval, recompute skip list, run dialectical
  audit, blind A/B snapshot, APK build link, live telemetry.
- DeployTab component inlined at the bottom of app/dev/page.js,
  reusing /dev's css + token conventions. Tab switch fetches
  /api/stats on mount.

One canonical ops entrypoint again.