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.

.github/workflows/gatekeep-pr.yml

@@ -3,6 +3,12 @@
 #
 # Secret: OPENROUTER_API_KEY (already configured on this repo)
 # Action: in-repo composite at repository root (uses: ./)
+#
+# Important (uses: ./):
+# - Checkout is the PR head, so Action fixes on main do NOT apply until the
+#   PR branch rebases/merges main. Stale heads can look "broken" after a fix.
+# - Re-run: push an empty commit, or Actions → this workflow → Run workflow
+#   (workflow_dispatch; uses default branch / manual inputs as configured).
 
 name: Gatekeep