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/mod.rs

@@ -161,6 +161,42 @@ pub fn pipeline_stage(agent_name: &str) -> PipelineStage {
     }
 }
 
+/// Map a pipeline [`Stage`] to the canonical [`PipelineStage`] for LLM agent spawning.
+///
+/// Returns `None` for stages where no LLM agent should be active (terminal states,
+/// blocked, frozen, or unclassified merge failures requiring human intervention).
+/// Returns `Some(stage)` naming the single LLM-agent type that may run on this story.
+/// Used by `validate_agent_stage` and `reconcile_canonical_agents` to enforce the
+/// one-agent-per-story invariant (story 1100).
+pub fn canonical_pipeline_stage(s: &crate::pipeline_state::Stage) -> Option<PipelineStage> {
+    use crate::pipeline_state::{MergeFailureKind, Stage};
+    match s {
+        Stage::Coding { .. } => Some(PipelineStage::Coder),
+        Stage::Qa => Some(PipelineStage::Qa),
+        Stage::Merge { .. } => Some(PipelineStage::Mergemaster),
+        Stage::MergeFailure {
+            kind: MergeFailureKind::ConflictDetected(_),
+            ..
+        } => Some(PipelineStage::Mergemaster),
+        Stage::MergeFailure {
+            kind: MergeFailureKind::GatesFailed(_),
+            ..
+        } => Some(PipelineStage::Coder),
+        Stage::MergeFailureFinal { .. } => Some(PipelineStage::Mergemaster),
+        Stage::Upcoming
+        | Stage::Backlog
+        | Stage::MergeFailure { .. }
+        | Stage::Done { .. }
+        | Stage::Blocked { .. }
+        | Stage::Archived { .. }
+        | Stage::Frozen { .. }
+        | Stage::ReviewHold { .. }
+        | Stage::Abandoned { .. }
+        | Stage::Superseded { .. }
+        | Stage::Rejected { .. } => None,
+    }
+}
+
 /// Determine the pipeline stage for a configured agent.
 ///
 /// Prefers the explicit `stage` config field (added in Bug 150) over the