ci/manifest enforcement

[sdcarter]

ci: add manifest enforcement via pre-commit and CI

Summary

Adds automation to ensure data/manifest.json stays in sync with car data files without creating noise commits in main.

Motivation

Currently, contributors must manually run scripts/generate_manifest.py before committing. If they forget, the manifest becomes stale. Rather than having CI auto-commit changes (which creates extra commits in main), this PR enforces that the manifest is always up-to-date at PR-time.

What's included

1. Pre-commit hook (.pre-commit-config.yaml)

   • Automatically runs generate_manifest.py on every commit
   • Stages data/manifest.json if it changes
   • Ensures developers who have pre-commit installed never forget to update the manifest

2. CI check (.github/workflows/manifest-check.yml)

   • Runs on PRs and pushes to main that touch scripts/** or data/**
   • Regenerates the manifest and fails if it differs from the committed version
   • Prevents merging PRs with stale manifests

How it works

• Local (pre-commit users): Manifest auto-generates and stages on commit
• CI (everyone): Pipeline fails if manifest is out-of-date, requiring the contributor to regenerate and push an update

This keeps main clean by requiring manifest updates within the PR rather than having CI create follow-up commits.

Testing

• Added/removed car files trigger manifest regeneration
• CI check passes when manifest is current, fails when stale
• Pre-commit hook runs cleanly without modifying unrelated files