feat: add multi-select ask-user prompts

[corwinm]

Summary

• Add a distinct multiSelect option for ask_user_question when callers need multiple predefined choices.
• Return mode: "multi-select" and set answer to a string array for selected options.
• Keep existing single-choice options behavior unchanged and covered by regression tests.
• Document the new multi-select capability and add a changeset.

Test Plan

• npm run test -- extensions/pi-ask-user-question/test/index.test.ts
• npm run ci