Parse @agentname patterns in channel message body. Mentioned agents
(who are channel members, excluding sender) get mention=true flag in
their inbox notification metadata. Channel message metadata includes
mentioned_agents list for all recipients.
- mentions.go: regex parser with email exclusion, dedup, 22 test cases
- BroadcastMessage: metadata now uses json.Marshal, includes mentions
- Tests verify mention flag, self-mention exclusion, non-member skip
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
