- Add MessageType enum (task, scavenge, notification, reply)
- Expand Priority from 2 to 4 levels (low, normal, high, urgent)
- Add ThreadID and ReplyTo fields to Message struct
- Add --type and --reply-to flags to 'gt mail send'
- Add 'gt mail thread <id>' command to view conversation threads
- Update inbox/read display to show type and threading info
- Auto-generate thread IDs for new messages
- Reply messages inherit thread from original
Closes: gt-hgk
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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