Devcontainer build logs should be nested inside the devcontainer block

## Problem

Currently, devcontainer build logs appear in the parent agent's log panel. This makes it hard for users to infer where these logs will appear and creates a confusing UX.

### Current Issues
- Devcontainer logs are mixed with parent agent startup logs
- Logs can hit the 1MB limit, preventing further updates when used actively
- The current approach was implemented under time pressure and needs revisiting

## Proposed Solution

Separate devcontainer build logs into a nested devcontainer block instead of displaying them in the parent agent's log panel.

## Additional Context

The current implementation has a nice property where deleting and re-creating sub-agents occasionally resets the 1MB limit, allowing logs to continue. Any new design should consider how to handle log limits gracefully.