feat(gate): add add-waiter and show commands for phase handoff

- Add `bd gate add-waiter <gate-id> <waiter>` command to register
  agents as waiters on a gate bead using the native Waiters field
- Add `bd gate show <gate-id>` command to display gate details
  including waiters (used by gt gate wake)
- Add Waiters field to UpdateArgs in RPC protocol
- Update server to handle waiters field in updates

This is part of the polecat phase handoff feature (bd-quw1).
The corresponding gastown changes (gt done --phase-complete) are
tracked separately.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/rpc/server_issues_epics.go

@@ -135,10 +135,13 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.EventPayload != nil {
 		u["event_payload"] = *a.EventPayload
 	}
-	// Gate fields (bd-z6kw: support await_id updates for gate discovery)
+	// Gate fields
 	if a.AwaitID != nil {
 		u["await_id"] = *a.AwaitID
 	}
+	if len(a.Waiters) > 0 {
+		u["waiters"] = a.Waiters
+	}
 	return u
 }