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_group_test.go

@@ -163,9 +163,9 @@ func TestGroupBeadID(t *testing.T) {
 		name string
 		want string
 	}{
-		{"ops-team", "gt-group-ops-team"},
-		{"all", "gt-group-all"},
-		{"crew-leads", "gt-group-crew-leads"},
+		{"ops-team", "hq-group-ops-team"},
+		{"all", "hq-group-all"},
+		{"crew-leads", "hq-group-crew-leads"},
 	}
 
 	for _, tt := range tests {