fix(core): ignore generated dirs in nested discovery [#437]

[intellectronica]

Fixes #437.

Summary

• Add a default ignore set for generated, fixture, dependency, and temp directories during nested .ruler discovery.
• Preserve normal nested project discovery and nested git boundary behavior.
• Add regression coverage for ignored directories and normal nested configs.

Verification

• npm ci
• npm run lint
• npm test
• npm run build