[FEATURE] Sub-agent TUI output rendering order is reversed + request for configurable layout

Problem Statement

When navigating into a sub-agent's view in the TUI, the content is rendered in reverse order relative to what you'd expect from a normal terminal conversation:

Current rendering (top → bottom):

1. Main agent output
2. Sub-agent's current activity / live progress
3. Sub-agent's original prompt

Expected rendering (top → bottom):

1. Main agent output
2. Sub-agent's original prompt
3. Sub-agent's current activity / live progress

The terminal convention is that the most recent output is at the bottom. Having the prompt at the bottom and live progress above it means the sub-agent section reads upside-down relative to every other part of the interface.

Proposed Solution

1. Fix the default rendering order — sub-agent sections should render prompt-first, progress below, consistent with how every other part of the TUI works.

1. Expose a configurable layout option — allow users to customize how sub-agent output is displayed in settings.json, e.g.:

``json
{
"subagentDisplay": {
"showPrompt": true,
"promptPosition": "top"
}
}
``

Use Case

When monitoring a long-running background sub-agent, the bottom of the terminal window should always reflect what the agent is currently doing. Right now it shows the prompt, which is the least useful thing to have at the bottom.