fix(tui): display error message when subagent crashes

Previously when a subagent task crashed or failed, it was immediately
removed from the UI without displaying what error occurred. This made
debugging very difficult as users could see the task disappear but had
no information about what went wrong.

Changes:
- Add error_message field to SubagentTaskDisplay to store the error
- Update ToolEvent::Failed handler to set the error message and keep
  the failed subagent visible instead of immediately removing it
- Update render_subagent to display the error message in red when
  status is Failed
- Failed subagents are still cleaned up on the next conversation turn
  via clear_tool_calls which retains only non-terminal subagents

Author: factorydroid

cortex-tui/src/app.rs

@@ -272,6 +272,9 @@ pub struct SubagentTaskDisplay {
     pub started_at: Instant,
     /// Current todo items from the subagent (if any).
     pub todos: Vec<SubagentTodoItem>,
+    /// Error message if the subagent failed.
+    /// Stored separately so it can be displayed even when the task is removed.
+    pub error_message: Option<String>,
 }
 
 impl SubagentTaskDisplay {
@@ -294,6 +297,7 @@ impl SubagentTaskDisplay {
             output_preview: String::new(),
             started_at: Instant::now(),
             todos: Vec::new(),
+            error_message: None,
         }
     }
 

cortex-tui/src/runner/event_loop.rs

@@ -3419,12 +3419,23 @@ impl EventLoop {
                     format!("Error: {}", error),
                 );
 
-                // If this is a Task tool, remove the subagent from display
+                // If this is a Task tool, update subagent status to Failed with error message
+                // Keep the subagent visible so users can see what error occurred
                 if name == "Task" || name == "task" {
                     let session_id = format!("subagent_{}", id);
-                    self.app_state.remove_subagent(&session_id);
-                    // Stop delegation mode if no more active subagents
-                    if !self.app_state.has_active_subagents() {
+                    let error_clone = error.clone();
+                    self.app_state.update_subagent(&session_id, |task| {
+                        task.status = SubagentDisplayStatus::Failed;
+                        task.error_message = Some(error_clone);
+                        task.current_activity = "Failed".to_string();
+                    });
+                    // Stop delegation mode if no more active (non-failed) subagents
+                    let has_running_subagents = self
+                        .app_state
+                        .active_subagents
+                        .iter()
+                        .any(|t| !t.status.is_terminal());
+                    if !has_running_subagents {
                         self.app_state.streaming.stop_delegation();
                     }
                 }

cortex-tui/src/views/minimal_session.rs

@@ -413,8 +413,35 @@ impl<'a> MinimalSessionView<'a> {
             ),
         ]));
 
-        // Display todos if any - use ⎿ prefix for first, space for rest
-        if !task.todos.is_empty() {
+        // Display error message if task failed
+        if task.status == SubagentDisplayStatus::Failed {
+            if let Some(ref error_msg) = task.error_message {
+                lines.push(Line::from(vec![
+                    Span::styled("  ⎿ ", Style::default().fg(self.colors.text_muted)),
+                    Span::styled("Error: ", Style::default().fg(self.colors.error)),
+                ]));
+                // Display error message, truncate if too long
+                for (i, err_line) in error_msg.lines().take(5).enumerate() {
+                    let truncated = if err_line.len() > 70 {
+                        format!("{}...", &err_line.chars().take(67).collect::<String>())
+                    } else {
+                        err_line.to_string()
+                    };
+                    let prefix = if i == 0 { "    " } else { "    " };
+                    lines.push(Line::from(vec![
+                        Span::styled(prefix, Style::default().fg(self.colors.text_muted)),
+                        Span::styled(truncated, Style::default().fg(self.colors.error)),
+                    ]));
+                }
+            } else {
+                // Fallback: no error message provided
+                lines.push(Line::from(vec![
+                    Span::styled("  ⎿ ", Style::default().fg(self.colors.text_muted)),
+                    Span::styled("Task failed", Style::default().fg(self.colors.error)),
+                ]));
+            }
+        } else if !task.todos.is_empty() {
+            // Display todos if any - use ⎿ prefix for first, space for rest
             for (i, todo) in task.todos.iter().enumerate() {
                 let (status_text, status_color) = match todo.status {
                     SubagentTodoStatus::Completed => ("[completed]", self.colors.success),