fix: keep an adopted session off the direct-PTY fallback (release review)

A wrapper that could not start fell back to a direct PTY: `claude
--dangerously-skip-permissions` or a server-account shell in /tmp on this
host, under a tab still carrying the foreign session's name, so someone who
opened an ssh or container session ended up typing into this machine.

startInteractive and startShell now fail the start for an adopted session
and leave `_useMux` on, so selecting the tab tries the wrapper again. The
`error` event carries the reason, since the browser's re-attach does not
read the route's answer. A session with no mux at all is refused the same
way.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/session.ts

@@ -3205,6 +3205,26 @@ export class Session extends EventEmitter {
     this.emit('output', data);
   }
 
+  /**
+   * End a start that cannot reach an adopted session's wrapper.
+   *
+   * ⚠️ Never the direct-PTY fallback. What that starts is a NEW process on THIS host
+   * (`claude --dangerously-skip-permissions`, or a server-account shell) in the neutral
+   * `/tmp`, under a tab still carrying the foreign session's name, so someone who opened an
+   * ssh or container session would end up typing into this machine. `_useMux` stays on, so
+   * the next start, which is what selecting the tab sends, tries the wrapper again.
+   *
+   * The `error` event is what the tab shows: the browser's re-attach does not read the
+   * route's answer.
+   */
+  private _failAdoptedStart(err: unknown): never {
+    const message = `Could not attach to ${this._adopt?.targetSession}: ${getErrorMessage(err)}`;
+    console.error(`[Session] ${message}`);
+    this._status = 'stopped';
+    this.emit('error', message);
+    throw new Error(message);
+  }
+
   async startInteractive(): Promise<void> {
     if (this.ptyProcess) {
       throw new Error('Session already has a running process');
@@ -3441,6 +3461,7 @@ export class Session extends EventEmitter {
           this._armPaneSettle(true);
         }
       } catch (err) {
+        if (this._adopt) this._failAdoptedStart(err);
         console.error('[Session] Failed to create mux session, falling back to direct PTY:', err);
         this._useMux = false;
         this._muxSession = null;
@@ -3460,6 +3481,7 @@ export class Session extends EventEmitter {
       if (getCli(this.mode)?.capabilities.requiresMux) {
         throw new Error(`${getModeLabel(this.mode)} sessions require tmux. Direct PTY fallback is not supported.`);
       }
+      if (this._adopt) this._failAdoptedStart(new Error('tmux is not available'));
       try {
         // Pass --session-id to use the SAME ID as the Codeman session
         // This ensures subagents can be directly matched to the correct tab.
@@ -4463,6 +4485,7 @@ export class Session extends EventEmitter {
           }, 100);
         }
       } catch (err) {
+        if (this._adopt) this._failAdoptedStart(err);
         console.error('[Session] Failed to create mux session, falling back to direct PTY:', err);
         this._useMux = false;
         this._muxSession = null;
@@ -4471,6 +4494,7 @@ export class Session extends EventEmitter {
 
     // Fallback to direct PTY if mux is not used
     if (!this.ptyProcess) {
+      if (this._adopt) this._failAdoptedStart(new Error('tmux is not available'));
       try {
         this.ptyProcess = spawnPtyWithHelperRepair(() =>
           pty.spawn(shell, [], {