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/api/client.ts

@@ -57,7 +57,9 @@ export type WsResponse =
 			story_id: string;
 			status: string;
 			message: string;
-	  };
+	  }
+	/** `.story_kit/project.toml` was modified; re-fetch the agent roster. */
+	| { type: "agent_config_changed" };
 
 export interface ProviderConfig {
 	provider: string;
@@ -273,6 +275,7 @@ export class ChatWebSocket {
 		status: string,
 		message: string,
 	) => void;
+	private onAgentConfigChanged?: () => void;
 	private connected = false;
 	private closeTimer?: number;
 	private wsPath = DEFAULT_WS_PATH;
@@ -322,6 +325,7 @@ export class ChatWebSocket {
 						data.status,
 						data.message,
 					);
+				if (data.type === "agent_config_changed") this.onAgentConfigChanged?.();
 			} catch (err) {
 				this.onError?.(String(err));
 			}
@@ -367,6 +371,7 @@ export class ChatWebSocket {
 				status: string,
 				message: string,
 			) => void;
+			onAgentConfigChanged?: () => void;
 		},
 		wsPath = DEFAULT_WS_PATH,
 	) {
@@ -378,6 +383,7 @@ export class ChatWebSocket {
 		this.onPermissionRequest = handlers.onPermissionRequest;
 		this.onActivity = handlers.onActivity;
 		this.onReconciliationProgress = handlers.onReconciliationProgress;
+		this.onAgentConfigChanged = handlers.onAgentConfigChanged;
 		this.wsPath = wsPath;
 		this.shouldReconnect = true;