Add `gt convoy stranded` command to detect convoys with ready work but
no workers processing them. A convoy is stranded when it has open,
unblocked issues with no live assignee.
- New command outputs stranded convoy IDs with ready issue counts
- Supports --json for automation by Deacon patrol
- Checks blocked status via bd blocked
- Verifies assignee session liveness via tmux
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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