gt witness start and gt refinery start now detect zombie sessions
(tmux alive but Claude dead) and automatically kill and recreate them.
This makes the start commands idempotent:
- If no session exists → create new session
- If session exists and healthy → do nothing (already running)
- If session exists but zombie → kill and recreate
Previously users had to manually run stop then start, or use restart.
Closes: gt-ekc5u
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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