feat: add AskUserQuestion desktop flow

[Astro-Han]

Summary

• register PR2's AskUserQuestion tool only for the Desktop root agent
• add the validated main/preload/renderer response route
• replace the permission-only queue with one per-session FIFO shared by permissions and questions
• render one question at a time in the existing composer takeover, including descriptions, client-owned Other, Previous/Next, nullable answers, Stop, and one final submit
• cover the complete fake-backend same-turn continuation plus focused IPC, renderer, style, and keyboard-focus contracts

Scope

Desktop root-agent registration and UI only. This does not add the tool to headless/Harbor, Pi transport, or child agents, and it reuses the runtime protocol merged in PR #908.

Part of #825.

Screenshot

The screenshot is captured from the production UserQuestionPrompt Storybook story at 1280×720.

Verification

• npm run typecheck
• npm --workspace @maka/ui test — 134 passed
• npm --workspace @maka/runtime test — passed
• npm --workspace @maka/desktop test — 2474 passed
• npm --workspace @maka/desktop run e2e -- ask-user-question.spec.ts — 1 passed
• full Desktop Electron E2E suite — 27 passed during implementation
• check-console, check-a11y, and check-copy — passed

Review

Independent reviewer verdict: GO. The final implementation keeps one runtime protocol and one interaction queue; no P0–P3 findings remain after the keyboard-focus and screenshot-label fixes.