Fix: Remove relatedPosts field causing KeyStatic error

The relationship field was causing "n.call is not a function" error.
Removed relatedPosts from both keystatic.config.ts and content.config.ts.

Other optimizations remain:
- Site singleton for global settings
- Brand mark (globe logo)
- Entry layout grouping
- Preview URL configuration
- Published and featured checkbox fields

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

.env.example

@@ -0,0 +1,11 @@
+# KeyStatic GitHub Mode Environment Variables
+
+# GitHub OAuth Credentials (from GitHub App)
+KEYSTATIC_GITHUB_CLIENT_ID=your_client_id_here
+KEYSTATIC_GITHUB_CLIENT_SECRET=your_client_secret_here
+
+# Secret for signing cookies/tokens (generate a random string)
+KEYSTATIC_SECRET=your_random_secret_here
+
+# GitHub App Slug (needed for Astro)
+PUBLIC_KEYSTATIC_GITHUB_APP_SLUG=your_github_app_slug_here