Skip nested git repos when discovering .ruler directories

[Codex]

Parent repos in nested mode were incorrectly adding .gitignore entries for .ruler paths inside child git repos. Child repos should be isolated and not influence the parent’s ignore list.

• Traversal guard: findAllRulerDirs now stops descending into subdirectories that contain a .git directory (except the starting root), preventing cross-repo discovery.
• Coverage: Added a nested git test ensuring only sibling/root .ruler directories are returned when a child repo exists.

Example:

// Parent repo with nested child repo
const dirs = await findAllRulerDirs('/repo');
console.log(dirs);
// Before: ['/repo/sub/.ruler', '/repo/.ruler']
// Now:    ['/repo/.ruler']  // child git repo skipped

Original prompt

Fix #373 according to the plan in #373 (comment)