ci(docs): add pr build check for docs site

Splits workflow into build (runs on PRs) and deploy (runs on main push
only) so broken docs are caught before merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shep-ai[bot]

.github/workflows/docs.yml

@@ -7,13 +7,42 @@ on:
     paths:
       - 'docs/**'
       - '.github/workflows/docs.yml'
+  pull_request:
+    paths:
+      - 'docs/**'
+      - '.github/workflows/docs.yml'
   workflow_dispatch:
 
 permissions:
   contents: write
 
 jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: docs
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+          cache-dependency-path: docs/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build Docusaurus site
+        run: npm run build
+
   deploy:
+    needs: build
+    if: github.ref == 'refs/heads/main' && github.event_name == 'push'
     runs-on: ubuntu-latest
     defaults:
       run: