fix(tui): improve subagent display and timer tracking

[echobt]

Summary

This PR fixes several issues with the TUI when subagents (Task tool) are running:

Changes

1. Todo list visibility: Main agent and subagents are now instructed to use TodoWrite tool for real-time progress visibility

   • Added Todo List section to main system prompt requiring TodoWrite usage for multi-step tasks
   • Enabled TodoWrite/TodoRead tools for subagents (only Task is disabled to prevent recursion)
   • Updated subagent system prompt to require immediate TodoWrite usage

2. Delegation indicator: Shows 'Delegation' status when a subagent is running instead of 'Working'

   • Added is_delegating flag to StreamingState
   • Status indicator now shows 'Delegation' when subagents are active

3. Timer tracking: Shows total time from last user prompt instead of per-task timer

   • Added prompt_started_at that persists across streaming restarts
   • Uses prompt_elapsed_seconds() for consistent elapsed time display
   • Added full_reset() to clear timer only when entire conversation turn is complete

4. Task visibility: Keeps Task display visible after completion (cleared on next conversation turn)

   • Subagents are no longer removed from active_subagents on completion
   • They remain visible with Completed/Failed status
   • Cleared only when user sends a new message

Testing

• Cargo check passes with no errors
• Cargo fmt passes