feat: status_reason field for workflows + proper failure logging

- Add status_reason column to workflows table (migration)
- AgentUpdate::WorkflowStatus and WorkflowComplete carry reason
- Dispatch failure logs to execution_log with reason
- Worker disconnect marks orphaned workflows as failed with reason
- All status transitions now have traceable cause

src/agent.rs

@@ -158,15 +158,20 @@ impl AgentManager {
                         tracing::info!("Workflow {} dispatched to worker '{}'", workflow_id, name);
                     }
                     Err(e) => {
-                        tracing::error!("Failed to dispatch workflow {}: {}", workflow_id, e);
-                        let _ = sqlx::query("UPDATE workflows SET status = 'failed' WHERE id = ?")
-                            .bind(&workflow_id).execute(&self.pool).await;
-                        let _ = btx.send(WsMessage::WorkflowStatusUpdate {
-                            workflow_id,
-                            status: "failed".into(),
+                        let reason = format!("调度失败: {}", e);
+                        tracing::error!("Failed to dispatch workflow {}: {}", workflow_id, reason);
+                        let _ = sqlx::query("UPDATE workflows SET status = 'failed', status_reason = ? WHERE id = ?")
+                            .bind(&reason).bind(&workflow_id).execute(&self.pool).await;
+                        // Log to execution_log so frontend can show the reason
+                        let log_id = uuid::Uuid::new_v4().to_string();
+                        let _ = sqlx::query(
+                            "INSERT INTO execution_log (id, workflow_id, step_order, tool_name, tool_input, output, status, created_at) VALUES (?, ?, 0, 'system', 'dispatch', ?, 'failed', datetime('now'))"
+                        ).bind(&log_id).bind(&workflow_id).bind(&reason).execute(&self.pool).await;
+                        let _ = btx.send(WsMessage::StepStatusUpdate {
+                            step_id: log_id, status: "failed".into(), output: reason,
                         });
-                        let _ = btx.send(WsMessage::Error {
-                            message: format!("No worker available: {}", e),
+                        let _ = btx.send(WsMessage::WorkflowStatusUpdate {
+                            workflow_id, status: "failed".into(),
                         });
                     }
                 }
@@ -745,6 +750,7 @@ pub async fn run_step_loop(
                         let _ = update_tx.send(AgentUpdate::WorkflowStatus {
                             workflow_id: workflow_id.to_string(),
                             status: "waiting_user".into(),
+                            reason: String::new(),
                         }).await;
                         send_execution(update_tx, workflow_id, step_order, "ask_user", reason, reason, "waiting").await;
 
@@ -789,6 +795,7 @@ pub async fn run_step_loop(
                         let _ = update_tx.send(AgentUpdate::WorkflowStatus {
                             workflow_id: workflow_id.to_string(),
                             status: "executing".into(),
+                            reason: String::new(),
                         }).await;
 
                         let tool_msg = if feedback.is_empty() {
@@ -1084,6 +1091,7 @@ pub async fn run_agent_loop(
                             let _ = update_tx.send(AgentUpdate::WorkflowStatus {
                                 workflow_id: workflow_id.to_string(),
                                 status: "waiting_user".into(),
+                                reason: String::new(),
                             }).await;
                             send_execution(update_tx, workflow_id, 0, "plan_approval", "等待确认计划", "等待用户确认执行计划", "waiting").await;
 
@@ -1114,6 +1122,7 @@ pub async fn run_agent_loop(
                                 let _ = update_tx.send(AgentUpdate::WorkflowStatus {
                                     workflow_id: workflow_id.to_string(),
                                     status: "executing".into(),
+                                    reason: String::new(),
                                 }).await;
                                 // Stay in Planning phase, continue the loop
                                 continue;
@@ -1130,6 +1139,7 @@ pub async fn run_agent_loop(
                             let _ = update_tx.send(AgentUpdate::WorkflowStatus {
                                 workflow_id: workflow_id.to_string(),
                                 status: "executing".into(),
+                                reason: String::new(),
                             }).await;
                         }