Add KeyStatic CMS integration

Features added:
- KeyStatic Core and Astro integration installed
- Markdoc integration for content rendering
- keystatic.config.ts with blog post schema
- Local storage mode for content management
- Conditional loading via SKIP_KEYSTATIC env var
- Netlify build command updated to skip KeyStatic in production

KeyStatic Admin UI available at:
- Local: http://localhost:4321/keystatic
- Production: Disabled for security

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -21,12 +21,15 @@
     "fix:prettier": "prettier -w ."
   },
   "dependencies": {
+    "@astrojs/markdoc": "^0.15.10",
     "@astrojs/react": "^4.4.2",
     "@astrojs/rss": "^4.0.11",
     "@astrojs/sitemap": "^3.3.0",
     "@astrolib/analytics": "^0.6.1",
     "@astrolib/seo": "^1.0.0-beta.8",
     "@fontsource-variable/inter": "^5.2.5",
+    "@keystatic/astro": "^5.0.6",
+    "@keystatic/core": "^0.5.48",
     "@types/react": "^19.2.8",
     "@types/react-dom": "^19.2.3",
     "astro": "^5.7.3",