fix(tui): display subagent todos and prevent HTTP 413 payload errors

[echobt]

Summary

This PR addresses two issues when running tasks via the Task tool (subagent delegation):

Issue 1: Todo list not displaying during delegation

When a subagent is delegated a task, the user sees:

● Task research
  ⎿ Processing request...

✽ Delegation (1m 55s • Esc to interrupt)

But the subagent's todo list (with [pending], [in_progress], [completed] items) never appears.

Root cause: The subagent runs in a background task and executes tools directly via the registry. When it calls TodoWrite, the result is never sent back to the UI.

Fix: Detect TodoWrite calls in spawn_subagent, parse the todos from the arguments, and send a new ToolEvent::TodoUpdated event to the UI. The event handler then updates the SubagentTaskDisplay.todos field.

Issue 2: HTTP 413 Payload Too Large

After the subagent runs for a while, the user sees:

Backend error: HTTP 413 Payload Too Large from https://api.cortex.foundation: Failed to buffer the request body: length limit exceeded

Root cause: The subagent accumulates messages and tool results in its context. Large tool outputs (from Grep, Read, etc.) can cause the total payload to exceed backend limits.

Fix: Truncate tool outputs to 32KB maximum in the subagent's tool execution loop. A clear truncation message is appended so the model knows the output was truncated.

Changes

• cortex-tui/src/events.rs: Add ToolEvent::TodoUpdated variant
• cortex-tui/src/runner/event_loop.rs:
  • Detect TodoWrite calls and send TodoUpdated event
  • Truncate tool outputs > 32KB to prevent payload errors
  • Handle TodoUpdated event to update app_state