Accept story 34: Per-Project Agent Configuration and Role Definitions

Replace single [agent] config with multi-agent [[agent]] roster system.
Each agent has name, role, model, allowed_tools, max_turns, max_budget_usd,
and system_prompt fields that map to Claude CLI flags at spawn time.

- AgentConfig expanded with structured fields, validated at startup (panics
  on duplicate names, empty names, non-positive budgets/turns)
- Backwards-compatible: legacy [agent] format auto-wraps with deprecation warning
- AgentPool uses composite "story_id:agent_name" keys for concurrent agents
- agent_name added to AgentEvent variants, AgentInfo, start/stop/subscribe APIs
- GET /agents/config returns roster, POST /agents/config/reload hot-reloads
- POST /agents/start accepts optional agent_name, /agents/stop requires it
- SSE route updated to /agents/:story_id/:agent_name/stream
- Frontend: roster badges, agent selector dropdown, composite-key state
- Project root initialized to cwd at startup so config endpoints work immediately

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

frontend/src/components/Chat.test.tsx

@@ -475,7 +475,11 @@ describe("Chat review panel", () => {
 	it("fetches upcoming stories on mount and renders panel", async () => {
 		mockedWorkflow.getUpcomingStories.mockResolvedValueOnce({
 			stories: [
-				{ story_id: "31_view_upcoming", name: "View Upcoming Stories", error: null },
+				{
+					story_id: "31_view_upcoming",
+					name: "View Upcoming Stories",
+					error: null,
+				},
 				{ story_id: "32_worktree", name: null, error: null },
 			],
 		});