Accept story 30: Worktree-based agent orchestration

Add git worktree isolation for concurrent story agents. Each agent now
runs in its own worktree with setup/teardown commands driven by
.story_kit/project.toml config. Agents stream output via SSE and support
start/stop lifecycle with Pending/Running/Completed/Failed statuses.

Backend: config.rs (TOML parsing), worktree.rs (git worktree lifecycle),
refactored agents.rs (broadcast streaming), agents_sse.rs (SSE endpoint).
Frontend: AgentPanel.tsx with Run/Stop buttons and streaming output log.

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

frontend/src/api/agents.ts

@@ -0,0 +1,115 @@
+export type AgentStatusValue = "pending" | "running" | "completed" | "failed";
+
+export interface AgentInfo {
+	story_id: string;
+	status: AgentStatusValue;
+	session_id: string | null;
+	worktree_path: string | null;
+}
+
+export interface AgentEvent {
+	type: "status" | "output" | "agent_json" | "done" | "error" | "warning";
+	story_id?: string;
+	status?: string;
+	text?: string;
+	data?: unknown;
+	session_id?: string | null;
+	message?: string;
+}
+
+const DEFAULT_API_BASE = "/api";
+
+function buildApiUrl(path: string, baseUrl = DEFAULT_API_BASE): string {
+	return `${baseUrl}${path}`;
+}
+
+async function requestJson<T>(
+	path: string,
+	options: RequestInit = {},
+	baseUrl = DEFAULT_API_BASE,
+): Promise<T> {
+	const res = await fetch(buildApiUrl(path, baseUrl), {
+		headers: {
+			"Content-Type": "application/json",
+			...(options.headers ?? {}),
+		},
+		...options,
+	});
+
+	if (!res.ok) {
+		const text = await res.text();
+		throw new Error(text || `Request failed (${res.status})`);
+	}
+
+	return res.json() as Promise<T>;
+}
+
+export const agentsApi = {
+	startAgent(storyId: string, baseUrl?: string) {
+		return requestJson<AgentInfo>(
+			"/agents/start",
+			{
+				method: "POST",
+				body: JSON.stringify({ story_id: storyId }),
+			},
+			baseUrl,
+		);
+	},
+
+	stopAgent(storyId: string, baseUrl?: string) {
+		return requestJson<boolean>(
+			"/agents/stop",
+			{
+				method: "POST",
+				body: JSON.stringify({ story_id: storyId }),
+			},
+			baseUrl,
+		);
+	},
+
+	listAgents(baseUrl?: string) {
+		return requestJson<AgentInfo[]>("/agents", {}, baseUrl);
+	},
+};
+
+/**
+ * Subscribe to SSE events for a running agent.
+ * Returns a cleanup function to close the connection.
+ */
+export function subscribeAgentStream(
+	storyId: string,
+	onEvent: (event: AgentEvent) => void,
+	onError?: (error: Event) => void,
+): () => void {
+	const host = import.meta.env.DEV ? "http://127.0.0.1:3001" : "";
+	const url = `${host}/agents/${encodeURIComponent(storyId)}/stream`;
+
+	const eventSource = new EventSource(url);
+
+	eventSource.onmessage = (e) => {
+		try {
+			const data = JSON.parse(e.data) as AgentEvent;
+			onEvent(data);
+
+			// Close on terminal events
+			if (
+				data.type === "done" ||
+				data.type === "error" ||
+				(data.type === "status" && data.status === "stopped")
+			) {
+				eventSource.close();
+			}
+		} catch (err) {
+			console.error("Failed to parse agent event:", err);
+		}
+	};
+
+	eventSource.onerror = (e) => {
+		onError?.(e);
+		eventSource.close();
+	};
+
+	return () => {
+		eventSource.close();
+	};
+}