Always clean stale taps before creating new ones

deleteTap before createTap in all four call sites:
snapshot restore, cold boot, agent start, snapshot create.
Prevents "Device or resource busy" from leftover taps.

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

src/agent-manager.ts

@@ -268,6 +268,7 @@ export async function startAgent(
   // Setup network
   ensureBridge();
   ensureNat();
+  deleteTap(tapDevice); // clean stale tap from previous run
   createTap(tapDevice);
 
   // Boot VM