feat: add bd slot commands for agent bead slot management (gt-h5sza)

Add slot management commands:
- bd slot set <agent> <slot> <bead> - set slot (error if occupied)
- bd slot clear <agent> <slot> - clear slot
- bd slot show <agent> - show all slots

These enforce cardinality constraints for agent bead slots.

🤖 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

@@ -101,6 +101,13 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.Pinned != nil {
 		u["pinned"] = *a.Pinned
 	}
+	// Agent slot fields (gt-h5sza)
+	if a.HookBead != nil {
+		u["hook_bead"] = *a.HookBead
+	}
+	if a.RoleBead != nil {
+		u["role_bead"] = *a.RoleBead
+	}
 	return u
 }