fix: hand TTY to tmux via syscall.Exec in session at (gt-cuo)

runSessionAttach was calling tmux via exec.Command(...).Run() with
buffered stdout/stderr, so tmux saw pipes instead of a TTY and failed
with "open terminal failed: not a terminal".

Use attachToTmuxSession() (already used by crew at, handoff, mayor,
witness, refinery, deacon), which syscall.Exec's tmux on Unix so it
inherits the parent shell's controlling terminal directly. Windows
path uses the existing stdio-passthrough wrapper.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeremyschlatter-intern

internal/cmd/session.go

@@ -336,8 +336,18 @@ func runSessionAttach(cmd *cobra.Command, args []string) error {
 		return err
 	}
 
-	// Attach (this replaces the process)
-	return polecatMgr.Attach(polecatName)
+	running, err := polecatMgr.IsRunning(polecatName)
+	if err != nil {
+		return fmt.Errorf("checking session: %w", err)
+	}
+	if !running {
+		return polecat.ErrSessionNotFound
+	}
+
+	// Hand the terminal off to tmux via syscall.Exec so tmux inherits our
+	// controlling TTY directly. Running tmux as a subprocess with buffered
+	// stdio triggers "open terminal failed: not a terminal".
+	return attachToTmuxSession(polecatMgr.SessionName(polecatName))
 }
 
 // SessionListItem represents a session in list output.