fix: harden partial 429 success, fallbacks, zh errors

Retry HTTP 429 with backoff; keep models beyond first 3 as
fallbacks; degrade agreement when some models fail; force-high
risk only if zero usable responses. Clearer zh-CN errors/docs for
dogfood uses: ./ rebase + workflow_dispatch re-run.

README.md

@@ -22,6 +22,8 @@ AI-assisted PR review for **your own repos**: parallel multi-model review across
 
 `layue13/gatekeep` reviews **its own** PRs via [`.github/workflows/gatekeep-pr.yml`](.github/workflows/gatekeep-pr.yml) with `uses: ./` and `secrets.OPENROUTER_API_KEY`. Open a PR against `main` to see a review (Gitea Actions must have a runner — same pool as other `layue13` repos).
 
+Because dogfood uses `uses: ./`, **rebase/merge `main` into the PR branch** after Action fixes land, or the PR still runs the old Action. Re-run via new push or **Actions → Gatekeep → Run workflow**. Free-model 429s are retried; extras in `models:` after the first 3 are fallbacks; risk is force-high only if **zero** models succeed.
+
 Allowlists / Gitea-on-github.com limitations: [`docs/remote-uses.md`](docs/remote-uses.md).
 
 ## Providers YAML