feat(FN-2727): add node routing controls to CLI and dashboard

- Add CLI support for task node routing with create --node, task set-node, and task clear-node commands
- Extend settings command validation/output for defaultNodeId and unavailableNodePolicy and document new keys
- Surface task routing details in CLI task show plus dashboard Routing tab status/binding reason indicators
- Enhance Settings modal node routing UX with selected-node health status and add/refresh tests and changeset

docs/settings-reference.md

@@ -91,7 +91,13 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
 | `maxWorktrees` | `number` | `4` | Max git worktrees. |
 | `pollIntervalMs` | `number` | `15000` | Scheduler poll interval (ms). |
 | `heartbeatMultiplier` | `number` | `1` | Global multiplier applied to all agent heartbeat intervals. Configured from the Agents screen (not Settings). |
+| `defaultNodeId` | `string` | `undefined` | Optional project default node ID. When set, tasks without a per-task `nodeId` override are routed to this node. |
 | `unavailableNodePolicy` | `"block" \| "fallback-local"` | `"block"` | Routing policy when a selected node is unavailable/unhealthy. `"block"` stops execution until the node is healthy; `"fallback-local"` runs the task on the local node instead. Applies to both project-default node routing and per-task node overrides. |
+
+CLI usage:
+- `fn settings set defaultNodeId <node-id>`
+- `fn settings set unavailableNodePolicy <block|fallback-local>`
+
 | `groupOverlappingFiles` | `boolean` | `true` | Serialize execution when file scopes overlap. |
 | `overlapIgnorePaths` | `string[]` | `[]` | Optional project-relative file or directory paths to exclude from overlap blocking (for example `docs` or `generated/openapi.json`). Entries are trimmed, deduplicated, and must not be absolute or contain `..` traversal. |
 | `autoMerge` | `boolean` | `true` | Auto-finalize tasks from `in-review`. |