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/mux-interface.ts

@@ -155,4 +155,7 @@ export interface TerminalMultiplexer extends EventEmitter {
 
   /** Check if the multiplexer binary is available on the system */
   isAvailable(): boolean;
+
+  /** Check if a multiplexer session actually exists (process-level check, not just tracked) */
+  muxSessionExists(muxName: string): boolean;
 }