Fix gh-aw exact-version enlistment preflight #2129

The public Squad gh-aw quick start can report a successful pinned install while retaining an older binary, and gh-aw v0.89.22 may emit its version on stderr.

Update the canonical quick start and shipped enlistment mirrors to prove exactly v0.89.22, clean-reinstall the exact pin on mismatch, and fail before repository artifact generation if the version still cannot be verified. Add contract coverage for stderr output, an existing v0.89.21 install, exact success, and persistent mismatch.