fix(beads): add CreatedAt to group/channel creation, check channel status

- 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>

internal/beads/beads_channel.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"time"
 )
 
 // ChannelFields holds structured fields for channel beads.
@@ -146,6 +147,7 @@ func (b *Beads) CreateChannelBead(name string, subscribers []string, createdBy s
 		Subscribers: subscribers,
 		Status:      ChannelStatusActive,
 		CreatedBy:   createdBy,
+		CreatedAt:   time.Now().Format(time.RFC3339),
 	}
 
 	description := FormatChannelDescription(title, fields)