* fix: disable Claude co-author attribution by default
* fix: harden default co-author opt-out and correct the docs
Follow-up on the co-author default in this PR.
- Remove the existsSync/writeFileSync race in the installer settings write
(CodeQL js/file-system-race, high). A single guarded read now covers the
fresh-install case, and unreadable or non-object settings are left untouched.
- Respect `attribution` as an explicit user choice. It supersedes
`includeCoAuthoredBy` in Claude Code 2.1.x, so a user who configured it would
otherwise have had a dead key written into their settings.
- Share one opt-out rule via scripts/lib/claude-commit-attribution.js instead of
duplicating it across the installer and plugin setup.
- Update the git-workflow rule and its nine mirrors and translations, which
still told users ECC does not ship this setting.
We keep writing the deprecated `includeCoAuthoredBy` key rather than
`attribution`: unknown keys fail Claude Code settings validation, so writing
`attribution` would break users on older versions.
