- Add CreatedAt timestamp to CreateGroupBead() in beads_group.go
- Add CreatedAt timestamp to CreateChannelBead() in beads_channel.go
- Check channel status before sending in router.go sendToChannel()
- Reject sends to closed channels with appropriate error message
Closes: gt-yibjdm, gt-bv2f97
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>