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.

docs/es/rules/common/git-workflow.md

@@ -9,7 +9,7 @@
 
 Tipos: feat, fix, refactor, docs, test, chore, perf, ci
 
-Nota: Para desactivar la atribución de coautoría, configure `"includeCoAuthoredBy": false` en `~/.claude/settings.json`; Claude Code agrega `Co-Authored-By` de forma predeterminada y ECC no incluye esta configuración.
+Nota: Las instalaciones gestionadas por ECC configuran `"includeCoAuthoredBy": false` en `~/.claude/settings.json`, por lo que los commits no incluyen `Co-Authored-By` de forma predeterminada. Para conservar la atribución de Claude, configure `"includeCoAuthoredBy": true` o `attribution`; ECC nunca sobrescribe una elección explícita.
 
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