feat(story-115): hot-reload project.toml agent config without server restart

- Extend `WatcherEvent` to an enum with `WorkItem` and `ConfigChanged` variants
  so the watcher can distinguish between pipeline-file changes and config changes
- Watch `.story_kit/project.toml` at the project root (ignoring worktree copies)
  and broadcast `WatcherEvent::ConfigChanged` on modification
- Forward `agent_config_changed` WebSocket message to connected clients; skip
  pipeline state refresh for config-only events
- Add `is_config_file()` helper with unit tests covering root vs. worktree paths
- Accept `configVersion` prop in `AgentPanel` and re-fetch the agent roster
  whenever it increments
- Increment `agentConfigVersion` in `Chat` on receipt of `agent_config_changed`
  WS event via new `onAgentConfigChanged` handler in `ChatWebSocket`

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AgentPanel.tsx

@@ -79,7 +79,12 @@ function agentKey(storyId: string, agentName: string): string {
 	return `${storyId}:${agentName}`;
 }
 
-export function AgentPanel() {
+interface AgentPanelProps {
+	/** Increment this to trigger a re-fetch of the agent roster. */
+	configVersion?: number;
+}
+
+export function AgentPanel({ configVersion = 0 }: AgentPanelProps) {
 	const { hiddenRosterAgents } = useLozengeFly();
 	const [agents, setAgents] = useState<Record<string, AgentState>>({});
 	const [roster, setRoster] = useState<AgentConfigInfo[]>([]);
@@ -90,13 +95,16 @@ export function AgentPanel() {
 	const [editingEditor, setEditingEditor] = useState(false);
 	const cleanupRefs = useRef<Record<string, () => void>>({});
 
-	// Load roster, existing agents, and editor preference on mount
+	// Re-fetch roster whenever configVersion changes (triggered by agent_config_changed WS event).
 	useEffect(() => {
 		agentsApi
 			.getAgentConfig()
 			.then(setRoster)
 			.catch((err) => console.error("Failed to load agent config:", err));
+	}, [configVersion]);
 
+	// Load existing agents and editor preference on mount
+	useEffect(() => {
 		agentsApi
 			.listAgents()
 			.then((agentList) => {