feat: add external ask_user responder bridge

[Maverobot]

Summary

• Emit a normalized ask:prompt event when ask_user opens, with an in-process respond(result | null) callback for external UIs.
• Race the local TUI/RPC prompt with the external responder so the first answer wins and the tool still returns the normal ask_user result shape.
• Emit ask:answered / ask:cancelled after settlement so mirrored prompt cards can close or mark themselves resolved.
• Harden settlement paths after review: direct custom() returns now settle the bridge, empty-option RPC fallback uses direct input again, external responses are normalized/validated, and timeout/abort cleanup is explicit.
• Document the event contract and RPC-dialog cancellation limitation in the README.

Motivation

This is intended for same-process integrations that mirror ask_user prompts outside the local terminal (mobile app, chat bridge, web UI, etc.) while preserving the existing local TUI. Normal CLI/TUI usage remains unchanged.

Test plan

• bun test index.test.ts single-select-layout.test.ts → 66 pass / 0 fail
• npm run check → npm pack --dry-run succeeds