fix(tui): keep session context visible above the question prompt

[omryMen]

Issue for this PR

Closes #37173

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The question prompt grows to fit its content. Ask a long question and it covers the transcript, hiding the work the agent just did, which is the context you need to answer. #37173 describes where that leaves you: hit escape to read anything, which cancels the questions, then ask the agent to repeat itself. The body doesn't scroll either, so anything past the viewport is just gone.

So the panel is capped now, and the question body sits in a scrollbox. Options and controls live outside it with flexShrink={0} and stay put.

Capping it alone gives you the opposite problem. A one-line question reserves the whole budget and leaves a gap above the options, 14 rows for 1 row of text on a 40-row terminal. So maxHeight on the scrollbox is whichever is smaller, the budget or the rows the question actually renders.

Two things make that row count wrong if you skip them. The sidebar opens on its own past 120 columns and eats 42, so the question is rendering into a narrower column than dimensions().width implies (same math as contentWidth in session/index.tsx). And a long paragraph wraps over many rows, so the count divides by that width and rounds up, since wrapping breaks on whitespace and not at the exact boundary. Round down and you clip the body. Round up and the scrollbox has a little slack, which nobody notices.

The history reserve is a percentage, not a fixed row count. Fixed starved the body on short terminals.

Worth flagging: #37173 asks for a show/hide toggle. This goes the other way and keeps enough context on screen that there's nothing to toggle. Say the word if you'd rather have the explicit control.

How did you verify your code works?

Rendered QuestionPrompt with testRender at fixed sizes and read viewport.height and scrollHeight off the scrollbox. Marker line above the prompt to confirm history survives:

Terminal	Question	Viewport	Content	Scrolls
100×40	short	1	1	no
180×40	short, sidebar open	1	1	no
100×40	long	14	80	yes
180×40	long, sidebar open	14	40	yes
100×20	long	5	80	yes
180×40	multi-select	1	1	no

History, options and controls stayed on screen in all six. Before the change the two short cases reported a viewport of 14 for 1 row of content.

Then a real session: failing command, then a ~1200 word question. Error stays visible, body scrolls, options don't move. Short question sits right on top of its options with no gap.

bun typecheck passes in packages/tui.

One thing if this code ever moves. The new memos read multi() and are declared after it. Solid evaluates memos eagerly at setup, so pointing at a later const throws a temporal dead zone error at runtime and tsc says nothing. Found that the hard way.

Screenshots / recordings

Short question, after. Text sits directly on its options:

┃  Should I continue with the current implementation?
┃
┃  1. Continue
┃     Proceed with the current implementation
┃  2. Stop
┃     Halt the current implementation
┃  ↑↓ select  enter submit  esc dismiss

Long question, after. Failing command still visible above, scrollbar mark on the right, options pinned:

┃  $ ls /definitely-does-not-exist-layout-check
┃  ls: /definitely-does-not-exist-layout-check: No such file or directory

   → Asked 1 question

┃  Context for the decision: we are at the final go/no-go gate for the     ▀
┃  Helios production rollout, the coordinated release that moves the
┃  payments-adjacent surface of our platform from the legacy monolithic
┃  session stack onto the new distributed identity and storage
┃  ... (scrolls)
┃
┃  1. Approve rollout
┃  2. Pause and investigate
┃  3. Roll back
┃  ↑↓ select  enter submit  esc dismiss

Before the fix the short case had 13 blank rows sitting between the question text and option 1.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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