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/components/Chat.tsx

@@ -6,6 +6,7 @@ import { api, ChatWebSocket } from "../api/client";
 import type { ReviewStory, UpcomingStory } from "../api/workflow";
 import { workflowApi } from "../api/workflow";
 import type { Message, ProviderConfig, ToolCall } from "../types";
+import { AgentPanel } from "./AgentPanel";
 import { ChatHeader } from "./ChatHeader";
 import { GatePanel } from "./GatePanel";
 import { ReviewPanel } from "./ReviewPanel";
@@ -743,6 +744,8 @@ export function Chat({ projectPath, onCloseProject }: ChatProps) {
 						lastRefresh={lastUpcomingRefresh}
 						onRefresh={refreshUpcomingStories}
 					/>
+
+					<AgentPanel stories={upcomingStories} />
 				</div>
 			</div>