The readme-v1-frozen pre-commit hook uses language: fail, which
unconditionally fails when its files pattern matches any file in the
candidate set. Since CI runs pre-commit with --all-files, README.md
always matches and the hook always fails, breaking CI on main and all
PRs.

Fix this with a two-part approach:

1. Skip the readme-v1-frozen hook in CI via the SKIP env var (the same
   mechanism already used for no-commit-to-branch). The hook continues
   to work locally where pre-commit only checks staged files.

2. Add a standalone CI step that uses git diff to check whether a PR
   actually modifies README.md. This provides CI-level enforcement
   that the pre-commit hook cannot offer in --all-files mode.