fix: tmux remain-on-exit prevents session loss when Claude exits

Previously, when Claude exited inside tmux (exitCode=0 or crash), tmux
destroyed the entire session — losing buffer, history, and causing crash
loops on restart attempts. Now:

- Set remain-on-exit on for all tmux sessions
- Detect dead panes via #{pane_dead} instead of assuming session is gone
- Respawn dead panes with tmux respawn-pane -k (preserves session)
- Applied to both startInteractive and startShell paths

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mux-interface.ts

@@ -158,4 +158,10 @@ export interface TerminalMultiplexer extends EventEmitter {
 
   /** Check if a multiplexer session actually exists (process-level check, not just tracked) */
   muxSessionExists(muxName: string): boolean;
+
+  /** Check if the pane in a session is dead (command exited but remain-on-exit keeps it alive) */
+  isPaneDead(muxName: string): boolean;
+
+  /** Respawn a dead pane with a fresh command. Returns the new PID or null on failure. */
+  respawnPane(sessionId: string, workingDir: string, mode: 'claude' | 'shell', niceConfig?: NiceConfig, model?: string): Promise<number | null>;
 }