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>

.story_kit/project.toml

@@ -0,0 +1,16 @@
+[[component]]
+name = "frontend"
+path = "frontend"
+setup = ["pnpm install", "pnpm run build"]
+teardown = []
+
+[[component]]
+name = "server"
+path = "."
+setup = ["cargo check"]
+teardown = []
+
+[agent]
+command = "claude"
+args = []
+prompt = "Read .story_kit/README.md, then pick up story {{story_id}}"