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/main.rs

@@ -1,10 +1,12 @@
 mod agents;
+mod config;
 mod http;
 mod io;
 mod llm;
 mod state;
 mod store;
 mod workflow;
+mod worktree;
 
 use crate::agents::AgentPool;
 use crate::http::build_routes;