ci: content-build tests + --check gate

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HuHRPE7VfppUJEaoGBEUqZ

.github/workflows/content-build.yml

@@ -0,0 +1,19 @@
+name: content-build
+on:
+  push:
+    paths: ["content/**", "tools/content_build/**", ".github/workflows/content-build.yml"]
+  pull_request:
+    paths: ["content/**", "tools/content_build/**"]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - run: pip install -r tools/requirements-dev.txt
+      - name: Unit tests
+        run: PYTHONPATH=tools python -m pytest tools/content_build/tests -q
+      - name: Content freshness + validity
+        run: PYTHONPATH=tools python -m content_build --check