Adds `gt mail claim <queue-name>` command that:
- Lists unclaimed messages in a work queue
- Picks the oldest unclaimed message
- Verifies caller eligibility against workers patterns
- Claims the message by updating assignee and status
- Prints claimed message details
Includes tests for worker pattern matching.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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