feat: add number shortcut for custom responses

[aspiers]

Summary

Make the custom/freeform response behave like a numbered option. Users can now press its displayed number to open the freeform editor directly in both single- and multi-select prompts, while canned-answer number keys retain their existing behavior.

Changes

• Display the custom-response row with the next available option number.
• Open the freeform editor when that number is pressed in single- or multi-select mode.
• Preserve highlight-only number shortcuts for canned single-select answers.
• Document the shortcut and add regression coverage for rendering and input behavior.

Testing

• bun test — 86 passed, 0 failed
• npm run check
• LSP diagnostics — clean

Notes

The shortcut is limited to 1–9, matching the existing numbered-option behavior.