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>

server/src/http/agents_sse.rs

@@ -5,16 +5,16 @@ use poem::web::{Data, Path};
 use poem::{Body, IntoResponse, Response};
 use std::sync::Arc;
 
-/// SSE endpoint: `GET /agents/:story_id/stream`
+/// SSE endpoint: `GET /agents/:story_id/:agent_name/stream`
 ///
 /// Streams `AgentEvent`s as Server-Sent Events. Each event is JSON-encoded
 /// with `data:` prefix and double newline terminator per the SSE spec.
 #[handler]
 pub async fn agent_stream(
-    Path(story_id): Path<String>,
+    Path((story_id, agent_name)): Path<(String, String)>,
     ctx: Data<&Arc<AppContext>>,
 ) -> impl IntoResponse {
-    let mut rx = match ctx.agents.subscribe(&story_id) {
+    let mut rx = match ctx.agents.subscribe(&story_id, &agent_name) {
         Ok(rx) => rx,
         Err(e) => {
             return Response::builder()