fix(tui): display error message when subagent crashes

[echobt]

Summary

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

Testing

• When a subagent crashes, users will now see:

  ● Task code
    ⎿ Error: 
      <error message displayed in red>
  

• The error message is truncated at 5 lines and 70 characters per line to fit the display
• Failed subagents remain visible until the next conversation turn starts