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.

defaults/providers.json

@@ -0,0 +1,16 @@
+{
+  "_deprecated": "Use providers.yaml.",
+  "agreement_min": 2,
+  "providers": {
+    "openrouter": {
+      "key": "OPENROUTER_API_KEY",
+      "protocol": "openai",
+      "models": [
+        "google/gemma-4-31b-it:free",
+        "google/gemma-4-26b-a4b-it:free",
+        "cohere/north-mini-code:free",
+        "nvidia/nemotron-3-nano-omni-30b-a3b-reasoning:free"
+      ]
+    }
+  }
+}