ci(tests): run the Oh My Pi compatibility suite as a required check

tests/test-omp-compat.mjs skipped whenever omp was not on PATH, which
was always the case in CI, so 0.6.1 shipped an import that pi's compat
entrypoint provides but OMP's bundled pi-ai does not (#74).

Add an omp-compat job that installs @oh-my-pi/pi-coding-agent with Bun
and runs the suite with OMP_COMPAT_REQUIRED=1, which turns the skip into
a failure. Add a first phase that loads the extension through
`omp models -e`, the same loader `omp plugin install` validates with;
it fails on the 0.6.1 index.ts and passes with the runtime resolution.

.github/workflows/ci.yml

@@ -34,6 +34,33 @@ jobs:
       - run: npm ci
       - run: npm run format:check
 
+  # ──────────────────────────────────────────────────────────
+  # Oh My Pi host compatibility — real omp binary, mock API
+  # ──────────────────────────────────────────────────────────
+  omp-compat:
+    name: Oh My Pi compatibility
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+      - run: npm ci
+      - name: Install Oh My Pi
+        run: |
+          npm install -g @oh-my-pi/pi-coding-agent@latest
+          echo "OMP_BIN=$(npm prefix -g)/bin/omp" >> "$GITHUB_ENV"
+      - name: Verify omp starts
+        run: '"$OMP_BIN" --version'
+      - name: Run OMP compatibility suite
+        env:
+          OMP_COMPAT_REQUIRED: "1"
+        run: node tests/test-omp-compat.mjs
+
   # ──────────────────────────────────────────────────────────
   # Code-level vulnerability scanning (SAST)
   # ──────────────────────────────────────────────────────────