Previously, gt doctor --fix would kill workers whose spawning process had
exited, even though the Claude session was still running in tmux.
Now both identity_check.go and CleanStaleLocks check if the tmux session
exists before declaring a lock stale. A lock is only truly stale if BOTH
the PID is dead AND the session does not exist.
Also added ListSessionIDs() to tmux package to handle locks that store
session IDs (%N or $N format) instead of session names.
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