Implement static mailing list expansion for the mail system:
- Add list:name address syntax (e.g., list:oncall)
- Load lists from ~/gt/config/messaging.json
- Fan-out delivery: each list member gets their own message copy
- Clear error handling for unknown list names
- Add tests for list detection, parsing, and expansion
- Update gt mail send help text with list:name documentation
- Show recipients in output when sending to a list
Example:
gt mail send list:oncall -s "Alert" -m "System down"
# Expands to: mayor/, gastown/witness
# Creates 2 message copies
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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