fix: crash handling — save session, kill agent, distinct exit codes

Hardened shutdown/crash behavior:
- Browser disconnect exits with code 2 (distinct from crash code 1)
- emergencyCleanup kills agent subprocess and saves session state
- Clean shutdown saves session before exit (chat history persists)
- Clear user message on browser disconnect: "Run $B connect to reconnect"

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

browse/src/browser-manager.ts

@@ -242,12 +242,13 @@ export class BrowserManager {
       await this.newTab();
     }
 
-    // Browser disconnect handler
+    // Browser disconnect handler — exit code 2 distinguishes from crashes (1)
     if (this.browser) {
       this.browser.on('disconnected', () => {
         if (this.intentionalDisconnect) return;
-        console.error('[browse] Real browser disconnected.');
-        process.exit(1);
+        console.error('[browse] Real browser disconnected (user closed or crashed).');
+        console.error('[browse] Run `$B connect` to reconnect.');
+        process.exit(2);
       });
     }