Agents window Markdown editor: macOS word and line navigation use the wrong modifiers

Testing #326540

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: Insiders with the experimental Markdown editor
- OS Version: macOS
- Audited package revision: microsoft/vscode-packages@1d81418d61ad769f154579196a51a9364c105e2d

### Steps to Reproduce

1. Open a Markdown document containing `alpha beta gamma` in **Editing** mode.
2. Put the caret immediately after `beta`.
3. Try the standard macOS editing commands:
- Option+Left
- Option+Backspace
- Cmd+Left
- Cmd+Backspace
- Cmd+Up

### Expected

- Option+Left and Option+Backspace operate on the previous word.
- Cmd+Left and Cmd+Backspace operate to the start of the visual line.
- Cmd+Up moves to the start of the document.

### Actual

- Option+Left and Option+Backspace operate on one character.
- Cmd+Left and Cmd+Backspace operate on the previous word.
- Cmd+Up does not move the caret.

### Impact

The editor breaks platform muscle memory and differs from both native macOS text controls and VS Code's normal editor. Keyboard-driven users must slow down and can delete the wrong amount of text. This is especially disruptive for accessibility workflows that rely on predictable keyboard navigation.