fix: ensure content directory exists before assets symlink in CI

After removing astro-site/src/content/docs/ in the single-source
migration, the content directory may not exist in a fresh checkout.
Create it before symlinking assets.

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

Author: braboj

.github/workflows/deploy.yml

@@ -33,8 +33,7 @@ jobs:
         run: npm ci
 
       - name: Create assets symlink
-        working-directory: astro-site/src/content
-        run: ln -s ../../../assets assets
+        run: mkdir -p astro-site/src/content && ln -s ../../../assets astro-site/src/content/assets
 
       - name: Build site
         working-directory: astro-site