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/server_issues_epics.go

@@ -142,6 +142,10 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if len(a.Waiters) > 0 {
 		u["waiters"] = a.Waiters
 	}
+	// Slot fields
+	if a.Holder != nil {
+		u["holder"] = *a.Holder
+	}
 	return u
 }