Implements @group address resolution for mail routing:
- @rig/<rigname>: All agents in a rig
- @town: All town-level agents (mayor, deacon)
- @witnesses, @dogs, @refineries: Role-based groups
- @crew/<rig>, @polecats/<rig>: Role+rig scoped groups
- @overseer: Human operator (uses overseer.json)
Resolution uses `bd list --type=agent` queries with
description filtering. Fan-out at send time creates
individual messages for each resolved recipient.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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