feat(FN-2721): enforce node override guard across task workflows

- Add a core node-override-guard module, export it from @fusion/core, and enforce conflicts in store updates
- Add API route and CLI extension safeguards so nodeId override updates are blocked when ownership would conflict
- Wire node override routing and validation through dashboard quick-create, list, modal, settings, and task form/detail surfaces
- Add focused unit and integration tests for core guard logic, workflow routes, and dashboard node override UX

packages/cli/skill/fusion/references/extension-tools.md

@@ -29,6 +29,7 @@ Update fields on an existing task. Supports modifying the title, description, de
 | `description` | string | — | New task description |
 | `depends` | array | — | New dependency list — replaces existing dependencies (e.g. ['FN-001', 'FN-002']) |
 | `agentId` | union | — | Agent ID to assign this task to, or null to clear (e.g. 'agent-abc123') |
+| `nodeId` | union | — | Node ID override for this task, or null to clear |
 
 ### fn_task_list