Add fleet registry removal with confirm modal and agent_removed WS.

Operators can remove machines from Crucible and dashboard rosters with honest messaging that deletion is registry-only; bulk select, oath ledger entries, and Go/Vitest coverage included.

server/web/src/context/WebSocketProvider.tsx

@@ -164,6 +164,7 @@ export function WebSocketProvider({ children }: { children: React.ReactNode }) {
             );
             break;
           }
+          case 'agent_removed':
           case 'agent_deleted': {
             const { agent_id } = msg.payload as { agent_id: string };
             setAgents((prev) => prev.filter((a) => a.id !== agent_id));