Fix MCP server startup for Claude Code compatibility

- Suppress stderr output (Claude Code rejects servers with stderr)
- Make SyncEngine.start() non-blocking (blocking sync caused 15s+ timeout)
- Skip Content-Length header lines gracefully (NDJSON transport)
- Silently ignore malformed JSON lines instead of sending error responses

src/claude_memory/sync.py

@@ -87,14 +87,7 @@ class SyncEngine:
         return self._last_sync_success
 
     def start(self) -> None:
-        """Run initial sync (blocking), then start background thread."""
-        try:
-            self._sync_once()
-            self._last_sync_success = True
-        except Exception:
-            logger.warning("Initial sync failed, starting in offline mode")
-            self._last_sync_success = False
-
+        """Start background sync thread (non-blocking)."""
         self._thread = threading.Thread(target=self._sync_loop, daemon=True)
         self._thread.start()