chore(ci): establish PR governance for agent collaboration (#177)

* chore(ci): establish PR governance for agent collaboration

* docs: add AGENTS playbook and strengthen agent collaboration workflow

---------

Co-authored-by: chumyin <183474434+chumyin@users.noreply.github.com>

.github/workflows/workflow-sanity.yml

@@ -0,0 +1,63 @@
+name: Workflow Sanity
+
+on:
+  pull_request:
+    paths:
+      - ".github/workflows/**"
+      - ".github/*.yml"
+      - ".github/*.yaml"
+  push:
+    branches: [main]
+    paths:
+      - ".github/workflows/**"
+      - ".github/*.yml"
+      - ".github/*.yaml"
+
+concurrency:
+  group: workflow-sanity-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  no-tabs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Fail on tabs in workflow files
+        shell: bash
+        run: |
+          set -euo pipefail
+          python - <<'PY'
+          from __future__ import annotations
+
+          import pathlib
+          import sys
+
+          root = pathlib.Path(".github/workflows")
+          bad: list[str] = []
+          for path in sorted(root.rglob("*.yml")):
+            if b"\t" in path.read_bytes():
+              bad.append(str(path))
+          for path in sorted(root.rglob("*.yaml")):
+            if b"\t" in path.read_bytes():
+              bad.append(str(path))
+
+          if bad:
+            print("Tabs found in workflow file(s):")
+            for path in bad:
+              print(f"- {path}")
+            sys.exit(1)
+          PY
+
+  actionlint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Lint GitHub workflows
+        uses: rhysd/actionlint@v1