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.

scripts/lib/claude-commit-attribution.js

@@ -0,0 +1,43 @@
+'use strict';
+
+// Claude Code appends a `Co-Authored-By` trailer to commits and PRs unless the
+// user opts out, so ECC-managed installs default that off.
+//
+// Two settings control the trailer. `attribution: { commit, pr }` is the current
+// one and wins when set; `includeCoAuthoredBy` is deprecated as of Claude Code
+// 2.1.x but still honored, and is the only one older versions understand. We
+// write the deprecated key because unknown keys fail settings validation, so
+// writing `attribution` would break users on older Claude Code. Either key being
+// present counts as a deliberate user choice that ECC must not overwrite.
+const COAUTHOR_SETTING_KEY = 'includeCoAuthoredBy';
+
+function hasExplicitCommitAttributionPreference(settings) {
+  if (!settings || typeof settings !== 'object') {
+    return false;
+  }
+  if (typeof settings[COAUTHOR_SETTING_KEY] === 'boolean') {
+    return true;
+  }
+
+  const attribution = settings.attribution;
+  return Boolean(attribution)
+    && typeof attribution === 'object'
+    && !Array.isArray(attribution)
+    && (attribution.commit !== undefined || attribution.pr !== undefined);
+}
+
+function withCommitAttributionDisabled(settings) {
+  if (hasExplicitCommitAttributionPreference(settings)) {
+    return settings;
+  }
+  return {
+    ...settings,
+    [COAUTHOR_SETTING_KEY]: false,
+  };
+}
+
+module.exports = {
+  COAUTHOR_SETTING_KEY,
+  hasExplicitCommitAttributionPreference,
+  withCommitAttributionDisabled,
+};