my-skills-standards-update — roll a new baseline into consumer repos

A branch of my-skills-maintenance: follow its shared rules (repo anchor $REPO, propose-before-write, verify-then-report, GLOSSARY vocabulary, the finishing proposal). The mandatory stray check does not apply to this branch — it touches neither skills/ nor the store.

The update path for CODING_STANDARDS.md files already generated in other repos. When setup-coding-standards' baseline gains a version, bring each target file up to it: replace the baseline region above the file's marker with the new baseline, keep everything below the marker untouched, show the diff, then write. Creation is setup-coding-standards' job (it stops when a file exists); this branch is that existing file's update path.

1. Collect target repos

Take the repo list from the user, or scan a directory they name (one level deep, looking for CODING_STANDARDS.md). Repos without that file are skipped — creating one is setup-coding-standards' job.