✨ feat: Remove --project-root, auto-detect from config file location

[heumsi]

Summary

• Remove --project-root CLI argument
• Auto-discover config file by searching upward from cwd (.python-dependency-linter.yaml, pyproject.toml)
• Use config file's parent directory as project root (like ruff/pytest)
• Add find_config() to config.py with upward directory traversal
• Extract shared TOML loading helper to reduce duplication

Test plan

• 54 tests pass (uv run pytest -v)
• ruff lint and format clean
• Auto-discovery: YAML in cwd, YAML in parent, pyproject.toml, YAML preferred over TOML
• --config explicit path uses config file's parent as root
• Config not found returns exit code 2 with descriptive error

🤖 Generated with Claude Code