refactor(flowctl): remove duplicate cycle detection from DAG construction [fn-110.4]

Move cycle detection out of from_tasks() — callers now call detect_cycles()
explicitly. This eliminates the redundant Kahn's algorithm run during DAG
construction. Updated stats.rs to add explicit cycle check.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: z23cc

flowctl/crates/flowctl-cli/src/commands/stats.rs

@@ -353,7 +353,12 @@ pub fn cmd_dag(json_flag: bool, epic_id: Option<String>) {
     };
 
     let dag = match flowctl_core::TaskDag::from_tasks(&tasks) {
-        Ok(d) => d,
+        Ok(d) => {
+            if let Some(cycle) = d.detect_cycles() {
+                error_exit(&format!("Cycle detected in DAG: {}", cycle.join(" -> ")));
+            }
+            d
+        }
         Err(e) => error_exit(&format!("Failed to build DAG: {}", e)),
     };
 

flowctl/crates/flowctl-core/src/dag.rs

@@ -67,15 +67,7 @@ impl TaskDag {
             }
         }
 
-        let dag = TaskDag { graph, index };
-
-        // Phase 3: cycle detection.
-        if let Some(cycle) = dag.detect_cycles() {
-            let cycle_str = cycle.join(" -> ");
-            return Err(CoreError::CycleDetected(cycle_str));
-        }
-
-        Ok(dag)
+        Ok(TaskDag { graph, index })
     }
 
     /// Return task IDs whose dependencies are all satisfied.
@@ -552,8 +544,9 @@ mod tests {
             make_task("b", &["a"]),
             make_task("c", &["b"]),
         ];
-        let err = TaskDag::from_tasks(&tasks).unwrap_err();
-        assert!(matches!(err, CoreError::CycleDetected(_)));
+        // from_tasks no longer detects cycles — callers must use detect_cycles()
+        let dag = TaskDag::from_tasks(&tasks).unwrap();
+        assert!(dag.detect_cycles().is_some());
     }
 
     // ── ready_tasks ─────────────────────────────────────────────────