Spike 3: Sub-agent infrastructure fixes for multi-agent coordination

- Fix CLAUDECODE env var blocking nested Claude Code sessions
- Add drain-based event_log for reliable get_agent_output polling
- Add non-SSE get_agent_output fallback (critical for MCP tool calls)
- Preserve worktrees on agent stop instead of destroying work
- Reap zombie processes with child.wait() after kill
- Increase broadcast buffer from 256 to 1024
- Engineer supervisor and coder prompts in project.toml
- Point .mcp.json to test port 3002

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

.story_kit/project.toml

@@ -7,34 +7,55 @@ teardown = []
 [[component]]
 name = "server"
 path = "."
-setup = ["cargo check"]
+setup = ["mkdir -p frontend/dist", "cargo check"]
 teardown = []
 
 [[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."
+max_turns = 200
+max_budget_usd = 15.00
+prompt = """You are the supervisor for story {{story_id}}. Your job is to coordinate coder agents to implement this story.
+
+## Your MCP Tools
+You have these tools via the story-kit MCP server:
+- start_agent(story_id, agent_name) - Start a coder agent on a story
+- get_agent_output(story_id, agent_name, timeout_ms) - Poll agent output (returns recent events, call repeatedly)
+- list_agents() - See all running agents and their status
+- stop_agent(story_id, agent_name) - Stop a running agent
+- get_story_todos(story_id) - Get unchecked acceptance criteria for a story in current/
+- ensure_acceptance(story_id) - Check if a story passes acceptance gates
+
+## Your Workflow
+1. Read the story file from .story_kit/stories/ to understand requirements
+2. Move it to current/ if it is in upcoming/
+3. Start coder-1 on the story: call start_agent with story_id="{{story_id}}" and agent_name="coder-1"
+4. Monitor progress: call get_agent_output every 30-60 seconds to check on the agent
+5. If the agent completes, review the worktree changes
+6. If the agent gets stuck or fails, stop it and start a fresh agent
+7. When the work looks complete, call ensure_acceptance to verify
+
+## Rules
+- Do NOT implement code yourself - delegate to coder agents
+- Only run one coder at a time per story
+- Focus on coordination, monitoring, and quality review"""
+system_prompt = "You are a supervisor agent. Use MCP tools to coordinate sub-agents. Never implement code directly - always delegate to coder agents and monitor their progress."
 
 [[agent]]
 name = "coder-1"
 role = "Full-stack engineer. Implements features across all components."
 model = "sonnet"
-max_turns = 30
+max_turns = 50
 max_budget_usd = 5.00
+prompt = "You are working in a git worktree on story {{story_id}}. Read CLAUDE.md first, then .story_kit/README.md to understand the dev process. Pick up the story from .story_kit/stories/ - move it to current/ if needed. Follow the SDTW process end-to-end. The worktree and feature branch already exist - do not create them. Check .mcp.json for MCP tools."
+system_prompt = "You are a full-stack engineer working autonomously in a git worktree. Follow the Story-Driven Test Workflow strictly. Run cargo clippy and biome checks before considering work complete. Do not coordinate with other agents - focus on your assigned story."
 
 [[agent]]
 name = "coder-2"
 role = "Full-stack engineer. Implements features across all components."
 model = "sonnet"
-max_turns = 30
+max_turns = 50
 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
+prompt = "You are working in a git worktree on story {{story_id}}. Read CLAUDE.md first, then .story_kit/README.md to understand the dev process. Pick up the story from .story_kit/stories/ - move it to current/ if needed. Follow the SDTW process end-to-end. The worktree and feature branch already exist - do not create them. Check .mcp.json for MCP tools."
+system_prompt = "You are a full-stack engineer working autonomously in a git worktree. Follow the Story-Driven Test Workflow strictly. Run cargo clippy and biome checks before considering work complete. Do not coordinate with other agents - focus on your assigned story."