Dogfood: run Gatekeep on this repo's PRs

Add .github/workflows/gatekeep-pr.yml with uses: ./ and
OPENROUTER_API_KEY from secrets. Forge-agnostic API helper for Gitea.

evals/cases/must-stay-low-typo.json

@@ -0,0 +1,14 @@
+{
+  "id": "must-stay-low-typo",
+  "description": "Tiny non-sensitive typo stays low",
+  "expect_min_tier": "low",
+  "expect_tier": "low",
+  "fixture": {
+    "title": "fix: typo",
+    "files": ["tools/format-notes.txt"],
+    "additions": 1,
+    "deletions": 1,
+    "ci_status": "success",
+    "diff": "diff --git a/tools/format-notes.txt b/tools/format-notes.txt\n"
+  }
+}