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>

.story_kit/project.toml

@@ -10,7 +10,31 @@ path = "."
 setup = ["cargo check"]
 teardown = []
 
-[agent]
-command = "claude"
-args = []
-prompt = "Read .story_kit/README.md, then pick up story {{story_id}}"
+[[agent]]
+name = "supervisor"
+role = "Coordinates work, reviews PRs, decomposes stories."
+model = "opus"
+max_turns = 50
+max_budget_usd = 10.00
+system_prompt = "You are a senior engineering lead. Coordinate the work, review code, and ensure quality."
+
+[[agent]]
+name = "coder-1"
+role = "Full-stack engineer. Implements features across all components."
+model = "sonnet"
+max_turns = 30
+max_budget_usd = 5.00
+
+[[agent]]
+name = "coder-2"
+role = "Full-stack engineer. Implements features across all components."
+model = "sonnet"
+max_turns = 30
+max_budget_usd = 5.00
+
+[[agent]]
+name = "reviewer"
+role = "Reviews code changes, runs tests, checks quality gates."
+model = "sonnet"
+max_turns = 20
+max_budget_usd = 3.00