Add a body line-length validator to scripts/check-commit. It
fails when a body line exceeds 76 characters, exempting
trailers, URLs, and unbreakable tokens. The 76 limit leaves
room for git log's four-space indent in an 80-column
terminal.
Align the subject limit with the documented 70 characters;
the checker allowed 90 before.
Document the rule as a hard, verifiable requirement in
AGENTS.md, CONTRIBUTING.md, the create-commit skill, and
the workflow memory, and point at scripts/check-commit.
Add tests for the validator and the subject length rule.
AI-assisted-by: deepseek-flash
