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>

server/src/http/mcp.rs

@@ -438,7 +438,7 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                 },
                 {
                     "name": "stop_agent",
-                    "description": "Stop a running agent and clean up its worktree.",
+                    "description": "Stop a running agent. Worktree is preserved for inspection.",
                     "inputSchema": {
                         "type": "object",
                         "properties": {
@@ -532,7 +532,7 @@ async fn handle_tools_call(
         "list_agents" => tool_list_agents(ctx),
         "get_agent_config" => tool_get_agent_config(ctx),
         "reload_agent_config" => tool_get_agent_config(ctx),
-        "get_agent_output" => Err("get_agent_output requires Accept: text/event-stream for SSE streaming".into()),
+        "get_agent_output" => tool_get_agent_output_poll(&args, ctx).await,
         _ => Err(format!("Unknown tool: {tool_name}")),
     };
 
@@ -737,6 +737,40 @@ fn tool_list_agents(ctx: &AppContext) -> Result<String, String> {
     .map_err(|e| format!("Serialization error: {e}"))
 }
 
+async fn tool_get_agent_output_poll(args: &Value, ctx: &AppContext) -> Result<String, String> {
+    let story_id = args
+        .get("story_id")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: story_id")?;
+    let agent_name = args
+        .get("agent_name")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: agent_name")?;
+
+    // Drain all accumulated events since the last poll.
+    let drained = ctx.agents.drain_events(story_id, agent_name)?;
+
+    let done = drained.iter().any(|e| {
+        matches!(
+            e,
+            crate::agents::AgentEvent::Done { .. } | crate::agents::AgentEvent::Error { .. }
+        )
+    });
+
+    let events: Vec<serde_json::Value> = drained
+        .into_iter()
+        .filter_map(|e| serde_json::to_value(&e).ok())
+        .collect();
+
+    serde_json::to_string_pretty(&json!({
+        "events": events,
+        "done": done,
+        "event_count": events.len(),
+        "message": if done { "Agent stream ended." } else if events.is_empty() { "No new events. Call again to continue." } else { "Events returned. Call again to continue." }
+    }))
+    .map_err(|e| format!("Serialization error: {e}"))
+}
+
 fn tool_get_agent_config(ctx: &AppContext) -> Result<String, String> {
     let project_root = ctx.agents.get_project_root(&ctx.state)?;
     let config = ProjectConfig::load(&project_root)?;