build: add CI workflow with production build step

Add .github/workflows/ci.yml that runs on every push and PR:
- Type check (pnpm astro check)
- Format check (pnpm run format:check)
- Agent markdown audit (--strict)
- Production build (pnpm run build)

This catches build regressions before merge.

Closes #644

Author: saif-at-scalekit

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+on:
+  push:
+    branches: ['**']
+  pull_request:
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'pnpm'
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Type check
+        run: pnpm astro check
+
+      - name: Format check
+        run: pnpm run format:check
+
+      - name: Agent markdown audit
+        run: node scripts/agent-markdown-audit.js --strict
+
+      - name: Production build
+        run: pnpm run build