feat(done): Add --phase-complete flag for gate-based phase handoffs

Add support for signaling phase completion when a polecat needs to wait
on a gate before continuing. The --phase-complete flag with --gate ID
allows polecats to hand off control while awaiting external conditions.

Changes:
- done.go: Add --phase-complete and --gate flags, PHASE_COMPLETE exit type
- protocol.go: Add Gate field to PolecatDonePayload
- handlers.go: Handle PHASE_COMPLETE by recycling session (keep worktree)
- beads.go: Add AddGateWaiter method for gate registration

This enables multi-phase molecule workflows with async coordination (bd-gxb4)

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

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

internal/beads/beads.go

@@ -1423,3 +1423,16 @@ func (b *Beads) FindMRForBranch(branch string) (*Issue, error) {
 
 	return nil, nil
 }
+
+// AddGateWaiter registers an agent as a waiter on a gate bead.
+// When the gate closes, the waiter will receive a wake notification via gt gate wake.
+// The waiter is typically the polecat's address (e.g., "gastown/polecats/Toast").
+func (b *Beads) AddGateWaiter(gateID, waiter string) error {
+	// Use bd gate add-waiter to register the waiter on the gate
+	// This adds the waiter to the gate's native waiters field
+	_, err := b.run("gate", "add-waiter", gateID, waiter)
+	if err != nil {
+		return fmt.Errorf("adding gate waiter: %w", err)
+	}
+	return nil
+}