feat(slot): Add merge-slot gate for serialized conflict resolution

Adds a new slot bead type and merge-slot commands for serializing
conflict resolution in the merge queue. This prevents "monkey knife
fights" where multiple polecats race to resolve conflicts.

- Add TypeSlot to bead types
- Add Holder field to Issue struct
- Add bd merge-slot create/check/acquire/release commands
- Add Holder field to UpdateArgs in RPC protocol

(gt-4u49x)

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

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

internal/rpc/protocol.go

@@ -164,6 +164,8 @@ type UpdateArgs struct {
 	// Gate fields
 	AwaitID *string  `json:"await_id,omitempty"` // Condition identifier for gates (run ID, PR number, etc.)
 	Waiters []string `json:"waiters,omitempty"`  // Mail addresses to notify when gate clears
+	// Slot fields
+	Holder *string `json:"holder,omitempty"` // Who currently holds the slot (for type=slot beads)
 }
 
 // CloseArgs represents arguments for the close operation