test: add config discovery vs autodiscovery test coverage

Add 5 comprehensive tests covering the interaction between config file
discovery (walking up directory tree) and autodiscovery mode triggering.

Tests added:
- Subdirectory with parent config (uses config, not autodiscovery)
- Subdirectory with empty parent config (triggers autodiscovery)
- Monorepo with multiple configs (merges configs correctly)
- Autodiscovery only when ALL arrays empty (edge case validation)
- No config anywhere in tree (autodiscovery from any depth)

These tests ensure the monorepo support and autodiscovery behavior
work correctly together, which was previously untested.

Author: Bishibop