feat: add /xrpc/_health endpoint

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Co-Authored-By: Claude Opus 4.5 <[e-mail]>

src/pds.js

···
  * - CAR file building for repo sync
  * - XRPC endpoints for repo operations and sync
  *
- * @see https://atproto.com/specs
+ * @see https://atproto.com
  */
 
 // === CONSTANTS ===
···
   ]
   if (repoWriteEndpoints.includes(url.pathname)) {
     return handleAuthenticatedRepoWrite(request, env)
+  }
+
+  // Health check endpoint
+  if (url.pathname === '/xrpc/_health') {
+    return Response.json({ version: '0.1.0' })
   }
 
   // Root path - ASCII art