[codex] Add prek sync-go-version hook

[simonwhitaker]

This change adds a prek pre-commit hook that runs tools/sync-go-version.sh before each commit so contributors catch mismatches between go.mod and the Docker build image locally instead of after pushing changes. Without this check, the repository can drift into a state where the declared Go version and the Dockerfile’s build image diverge, which makes local and CI builds less predictable. The root cause was that the sync script existed but was not wired into any default developer workflow, so it was easy to miss. The fix adds a local prek hook configuration and documents how to install hooks and run them manually in the contributor guide. Validation: prek run --all-files.