fix(beads): use hq- prefix for group/channel beads (town-level entities)

Groups and channels are town-level entities that span rigs, so they
should use the hq- prefix rather than gt- (rig-level).

Changes:
- GroupBeadID: gt-group- → hq-group-
- ChannelBeadID: gt-channel- → hq-channel-
- Add --force flag to bypass prefix validation (town beads may have
  mixed prefixes from test runs)
- Update tests and documentation

Also adds docs/beads-native-messaging.md documenting:
- New bead types (gt:group, gt:queue, gt:channel)
- CLI commands (gt mail group, gt mail channel)
- Address resolution logic
- Usage examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/beads/beads_channel.go

@@ -127,15 +127,15 @@ func ParseChannelFields(description string) *ChannelFields {
 	return fields
 }
 
-// ChannelBeadID returns the channel bead ID for a given channel name.
-// Format: hq-channel-<name> for town-level channels (default).
-// Town-level channels are stored in the shared beads database.
+// ChannelBeadID returns the bead ID for a channel name.
+// Format: hq-channel-<name> (town-level, channels span rigs)
 func ChannelBeadID(name string) string {
 	return "hq-channel-" + name
 }
 
 // CreateChannelBead creates a channel bead for pub/sub messaging.
-// The ID format is: hq-channel-<name> (e.g., hq-channel-alerts) for town-level.
+// The ID format is: hq-channel-<name> (e.g., hq-channel-alerts)
+// Channels are town-level entities (hq- prefix) because they span rigs.
 // The created_by field is populated from BD_ACTOR env var for provenance tracking.
 func (b *Beads) CreateChannelBead(name string, subscribers []string, createdBy string) (*Issue, error) {
 	id := ChannelBeadID(name)
@@ -156,6 +156,7 @@ func (b *Beads) CreateChannelBead(name string, subscribers []string, createdBy s
 		"--description=" + description,
 		"--type=task", // Channels use task type with gt:channel label
 		"--labels=gt:channel",
+		"--force", // Override prefix check (town beads may have mixed prefixes)
 	}
 
 	// Default actor from BD_ACTOR env var for provenance tracking