fix: detect stale tmux sessions on startInteractive/startShell

When Claude exits inside a tmux session, tmux destroys the session
(remain-on-exit is off), but the _muxSession reference persists.
On restart, startInteractive() treats it as a "restored session" and
tries to attach to the dead tmux — causing instant exit code 1.

Now checks tmux has-session before attaching. If the session is gone,
clears the stale reference and creates a fresh tmux session.

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

src/tmux-manager.ts

@@ -348,6 +348,10 @@ export class TmuxManager extends EventEmitter implements TerminalMultiplexer {
   /**
    * Check if a tmux session exists.
    */
+  muxSessionExists(muxName: string): boolean {
+    return this.sessionExists(muxName);
+  }
+
   private sessionExists(muxName: string): boolean {
     if (IS_TEST_MODE) return false;