huskies: merge 1100 bug Multiple LLM agents can run concurrently on the same story (coder + mergemaster + others) — enforce one-agent-per-story invariant

server/src/agents/pool/start/mod.rs

@@ -271,6 +271,42 @@ impl AgentPool {
                      '{conflicting_name}' is already active at the same pipeline stage"
                 ));
             }
+            // Cross-stage LLM agent guard: reject if any Coder/Qa/Mergemaster agent
+            // is already Running or Pending on this story at a *different* pipeline stage.
+            // These are stale agents left over from a previous stage transition that has
+            // since advanced. The periodic reconciler (reconcile_canonical_agents) stops
+            // them; here we surface the conflict so the caller waits for reconciliation.
+            if matches!(
+                resolved_stage,
+                PipelineStage::Coder | PipelineStage::Qa | PipelineStage::Mergemaster
+            ) && let Some(stale_name) = agents.iter().find_map(|(k, a)| {
+                let k_story = k.rsplit_once(':').map(|(s, _)| s).unwrap_or(k);
+                if k_story != story_id || a.agent_name == resolved_name {
+                    return None;
+                }
+                if !matches!(a.status, AgentStatus::Running | AgentStatus::Pending) {
+                    return None;
+                }
+                let a_stage = config
+                    .find_agent(&a.agent_name)
+                    .map(agent_config_stage)
+                    .unwrap_or_else(|| pipeline_stage(&a.agent_name));
+                if matches!(
+                    a_stage,
+                    PipelineStage::Coder | PipelineStage::Qa | PipelineStage::Mergemaster
+                ) && a_stage != resolved_stage
+                {
+                    Some(a.agent_name.clone())
+                } else {
+                    None
+                }
+            }) {
+                return Err(format!(
+                    "story '{story_id}' already has an active LLM agent '{stale_name}'; \
+                     refusing to spawn '{resolved_name}'"
+                ));
+            }
+
             // Enforce single-instance concurrency for explicitly-named agents:
             // if this agent is already running on any other story, reject.
             // Auto-selected agents are already guaranteed idle by