fix(tui): improve subagent display and timer tracking (#282)

Changes:
- Force main agent and subagents to use TodoWrite tool for progress visibility
- Show 'Delegation' status indicator when subagent is running
- Use total time from last user prompt instead of per-task timer
- Keep Task display visible after completion (cleared on next turn)
- Add prompt_started_at tracking for consistent elapsed time display
- Enable TodoWrite/TodoRead tools for subagents (only Task disabled)

Fixes issues with:
- Subagent todo list not visible in real-time
- Loading indicator not shown during agent delegation
- Timer resetting per task instead of showing total duration
- Task label disappearing immediately after completion

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt

cortex-tui/src/agent/system_prompt.rs

@@ -33,6 +33,13 @@ pub const SYSTEM_PROMPT_TEMPLATE: &str = r#"You are Cortex, an expert AI coding
 - For shell commands, explain what they do before executing
 - Call multiple tools in parallel when operations are independent
 
+# Todo List (IMPORTANT)
+For any non-trivial task that requires multiple steps:
+- Use the TodoWrite tool immediately to create a todo list tracking your progress
+- Update the todo list as you complete each step (mark items as in_progress or completed)
+- This provides real-time visibility to the user on what you're working on
+- Keep only ONE item as in_progress at a time
+
 # Guidelines
 - Always verify paths exist before operations
 - Handle errors gracefully and suggest alternatives

cortex-tui/src/app.rs

@@ -49,6 +49,11 @@ pub struct StreamingState {
     pub executing_tool: Option<String>,
     /// When the tool started executing (for elapsed time display)
     pub tool_started_at: Option<Instant>,
+    /// When the last user prompt was sent (for total elapsed time from user's perspective)
+    /// This persists across streaming restarts (e.g., after tool execution)
+    pub prompt_started_at: Option<Instant>,
+    /// Whether a subagent (Task) is currently running
+    pub is_delegating: bool,
 }
 
 impl Default for StreamingState {
@@ -61,6 +66,8 @@ impl Default for StreamingState {
             task_started_at: None,
             executing_tool: None,
             tool_started_at: None,
+            prompt_started_at: None,
+            is_delegating: false,
         }
     }
 }
@@ -71,6 +78,10 @@ impl StreamingState {
         self.thinking = true;
         self.current_tool = tool;
         self.task_started_at = Some(Instant::now());
+        // Only set prompt_started_at if not already set (first call in a turn)
+        if self.prompt_started_at.is_none() {
+            self.prompt_started_at = Some(Instant::now());
+        }
     }
 
     /// Get the elapsed seconds since the task started
@@ -80,13 +91,40 @@ impl StreamingState {
             .unwrap_or(0)
     }
 
+    /// Get the elapsed seconds since the original prompt was sent
+    /// This is the total time from the user's perspective
+    pub fn prompt_elapsed_seconds(&self) -> u64 {
+        self.prompt_started_at
+            .map(|t| t.elapsed().as_secs())
+            .unwrap_or(0)
+    }
+
     /// Reset streaming state when task completes
     pub fn stop(&mut self) {
         self.is_streaming = false;
         self.thinking = false;
         self.current_tool = None;
         self.tool_status = None;
         self.task_started_at = None;
+        // Note: prompt_started_at is NOT reset here - it persists until full_reset()
+    }
+
+    /// Full reset when the entire conversation turn is complete
+    /// (no more tool executions or continuations expected)
+    pub fn full_reset(&mut self) {
+        self.stop();
+        self.prompt_started_at = None;
+        self.is_delegating = false;
+    }
+
+    /// Start delegation mode (subagent is running)
+    pub fn start_delegation(&mut self) {
+        self.is_delegating = true;
+    }
+
+    /// Stop delegation mode
+    pub fn stop_delegation(&mut self) {
+        self.is_delegating = false;
     }
 
     /// Start tool execution in background
@@ -1347,6 +1385,8 @@ impl AppState {
     pub fn clear_tool_calls(&mut self) {
         self.tool_calls.clear();
         self.clear_content_segments();
+        // Also clear completed/failed subagents from the previous turn
+        self.active_subagents.retain(|t| !t.status.is_terminal());
     }
 
     /// Advance spinner frames for all running tool calls