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;
 }

src/session.ts

@@ -849,6 +849,12 @@ export class Session extends EventEmitter {
     // If mux wrapping is enabled, create or attach to a mux session
     if (this._useMux && this._mux) {
       try {
+        // Verify stale mux session — tmux destroys session when command exits
+        // (remain-on-exit is off), but _muxSession reference persists
+        if (this._muxSession && !this._mux.muxSessionExists(this._muxSession.muxName)) {
+          console.log('[Session] Stale mux session detected (tmux gone):', this._muxSession.muxName);
+          this._muxSession = null;
+        }
         // Check if we already have a mux session (restored session)
         const isRestoredSession = this._muxSession !== null;
         if (isRestoredSession) {
@@ -1128,6 +1134,11 @@ export class Session extends EventEmitter {
     // If mux wrapping is enabled, create or attach to a mux session
     if (this._useMux && this._mux) {
       try {
+        // Verify stale mux session — tmux destroys session when command exits
+        if (this._muxSession && !this._mux.muxSessionExists(this._muxSession.muxName)) {
+          console.log('[Session] Stale mux session detected (tmux gone):', this._muxSession.muxName);
+          this._muxSession = null;
+        }
         // Check if we already have a mux session (restored session)
         const isRestoredSession = this._muxSession !== null;
         if (isRestoredSession) {

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;