Summary

• Add provider model-change restriction handling so Grok keeps model changes on new threads when required.
• Add Grok ACP support for _x.ai/ask_user_question and return xAI's accepted-answer response shape.
• Remove the broken Grok exit-plan bridge and cover the supported xAI question shapes with focused tests.

Test plan

• bun run test src/provider/acp/XAiAcpExtension.test.ts src/provider/Layers/GrokAdapter.test.ts
• bun fmt
• bun lint (passes with one unrelated warning in CursorProvider.ts)
• bun typecheck
• Manual web UI flow with mock Grok ACP binary firing _x.ai/ask_user_question