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>

server/src/http/mod.rs

@@ -1,4 +1,5 @@
 pub mod agents;
+pub mod agents_sse;
 pub mod anthropic;
 pub mod assets;
 pub mod chat;
@@ -33,6 +34,10 @@ pub fn build_routes(ctx: AppContext) -> impl poem::Endpoint {
         .nest("/api", api_service)
         .nest("/docs", docs_service.swagger_ui())
         .at("/ws", get(ws::ws_handler))
+        .at(
+            "/agents/:story_id/stream",
+            get(agents_sse::agent_stream),
+        )
         .at("/health", get(health::health))
         .at("/assets/*path", get(assets::embedded_asset))
         .at("/", get(assets::embedded_index))