feat(issue-11026): Enable file reference autocomplete in question custom answer

[bezerraluiz]

What does this PR do?

Fixes #11026

In the TUI (planning mode), when the question tool shows the “Type your own answer” option, you can now insert file references like @path/to/file using the same autocomplete behavior as the main prompt input.

This fixes the case where the custom answer input couldn’t (or couldn’t easily) reference files during the question flow.

How did you verify your code works?

• Tests end-to-end

1. Start the TUI and enter a flow that triggers the question tool (e.g., planning mode).
2. Select “Type your own answer” to focus the textarea.
3. Type @ and start writing a path (e.g., @README.md).
4. Use autocomplete:
   • tab: selects the highlighted autocomplete item
   • esc: closes autocomplete
5. To submit the custom answer:
   • press enter again (with autocomplete closed) to confirm/advance, as before.

Screenshots:

[github-actions]

Hey! Your PR title Issue 11026 doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.