Cover base_config helper behavior

[codeforester]

Summary

• Add direct tests for safe_resolve covering existing paths, missing paths, and symlink targets.
• Add direct print_finding output-format coverage.
• Make base_config doctor report broken config symlinks distinctly.
• Add doctor coverage for broken symlink configs and valid YAML with unexpected top-level keys.

Validation

• PYTHONPATH=lib/python:cli/python "$HOME/.base.d/base/.venv/bin/python" -m pytest cli/python/base_config/tests/test_engine.py
• ./bin/base-test
• git diff --check

Fixes #304