TUI black-screen hang mid-turn in 1.0.70-0 (Windows Terminal); content intact, recoverable via --resume

### Describe the bug

Environment:
- CLI version: 1.0.70-0 (pre-release; auto-updated ahead of npm stable 1.0.69-1)
- Install: WinGet (GitHub.Copilot), self-updating pkg in %LOCALAPPDATA%\copilot\pkg\win32-x64
- OS: Windows 11, Windows Terminal
- Node: v24.15.0
- Extensions loaded: network-desk https://github.com/dmauser/network-desk

Symptom:
After a few prompts, the ENTIRE terminal viewport goes black and input freezes mid-turn.
The alt-screen renderer stops painting; keyboard input (Ctrl+L, Ctrl+O, Ctrl+C, resize)
does not recover the view. The tab still auto-titles from the response, so the backend
is producing output — only the live TUI is frozen.

Impact / proof content is intact:
Closing the terminal and running `copilot --resume` shows the FULL response for the
hung turn. No data loss — this is purely a client-side rendering/hang regression.
Logs show content_filter=0 and refusal=null, so it is not content filtering.

Regression window:
Started immediately after auto-update to 1.0.70-0. Earlier cached builds
(1.0.68 / 1.0.67) launched via `--prefer-version` do not exhibit the hang.

Workaround:
`copilot --prefer-version 1.0.68` (pins a good cached build and disables auto-update).
Recovery from a hung session: `copilot --resume`.

Suspected area:
Interactive alt-screen TUI renderer (enterAltScreen path) in 1.0.70-0.
Possibly aggravated by large per-turn additionalContext from an extension, but the
hang is the client renderer, not the model output.

### Affected version

GitHub Copilot CLI 1.0.70-0

### Steps to reproduce the behavior

1. Install extension using:
npx github:dmauser/network-desk init
2. run copilot in experimental mode:
copilot --experimental
3. Type on the copilot prompt:
@network-desk what are your capabilities?
We get the blank screen and it becomes unresponsive.
4. Kill the session and run
copilot --experimental --resume
5. Pick the one named: Explore Network Desk Capabilities.

### Expected behavior

The expected behavior is to get the answer on the same session screen without get the hang.

### Additional context

_No response_

Reproduce the hang with `copilot --experimental` and the `network-desk` extension using the reported prompt, then inspect the interactive alt-screen renderer around the mentioned `enterAltScreen` path. Compare 1.0.70-0 with 1.0.68 using `--prefer-version`; done means the response remains visible and input stays responsive without requiring `--resume`.

Written by the indexing model from the issue text.