[Feature Request] Diff review pane: add file collapse/expand, context expansion, and prompt line removal

**Bug Description**
The diff review pane could use some features. this is in the tui fullscreen mode

1. i can't interact with the files when it's open; i'd like to be able to collapse / expand different file diffs
2. when a new file is created, even after adding with `git add` it won't show the diff and says "New file not yet staged. Run git add ... to see line counts". i close the pane and reopen it to get it to refresh.
3. i'd like the ability to expand other lines in the review that aren't changes. for instance, maybe i'd like to click and expand the lines above a displayed diff
4. when i highlight lines in the diff and it injects them in my prompt, i can't then remove them from my prompt

**Environment Info**
- Platform: linux
- Terminal: xterm-256color
- Version: 2.1.270
- Feedback ID: 3f0d1fb5-504d-4187-98c5-c2f0a64f710f

**Errors**
```json
[]
```

Reproduce the diff review pane in TUI fullscreen mode on Linux with xterm-256color, including a newly created file after git add and highlighted diff lines inserted into the prompt. Trace the pane's file interaction and refresh behavior first. Done means files can collapse and expand, non-change context can expand, staged files refresh without reopening, and injected prompt lines can be removed.

Written by the indexing model from the issue text.