Long commit subjects and bullets are hard to read in narrow terminals and
mailed logs. Contributors using `.githooks` via core.hooksPath now get an
automatic guard in addition to the two-file pre-commit cap.
- Add `.githooks/commit-msg` to fail when any message line exceeds 80 chars
- Skip length checks while `.git/MERGE_HEAD` exists for merge commits
- Sync AGENTS.md, README.md, CONTRIBUTING.md, and commit.prompt.md
Jukka Aho