[ai] Extract build-queue dialog to standalone ask-build-queue script.

[ai] Extract build-queue dialog to standalone ask-build-queue script.

Move the AskUserQuestion dialog out of the guix-build Stop hook into
a separate script so it can be invoked proactively (before code changes
begin) rather than only at Stop time.

- ask-build-queue: new script with the build-target selection dialog.
  Exits early if a queue file already exists for the session.
- guix-build: simplified to use $CLAUDE_CODE_SESSION_ID directly
  instead of JSON parsing.  Delegates the no-queue case to
  ask-build-queue.
- CLAUDE.md: instructs to run ask-build-queue before any prompt that
  looks like it will modify code.

Co-Authored-By: Claude <noreply@anthropic.com>