test: add unit test infrastructure and tests for check.py

pytest.ini

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+[pytest]
+testpaths = tests
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*

requirements-dev.txt

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+pytest>=7.0.0
+pytest-cov>=4.0.0

tests/test_check.py

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+"""Unit tests for check.py script."""
+
+import sys
+from pathlib import Path
+
+# Add scripts directory to path for imports
+sys.path.insert(0, str(Path(__file__).parent.parent / "scripts"))
+
+
+def test_rel_function():
+    """Test the rel() function converts paths correctly."""
+    # Import after path is set up
+    import check
+
+    root = Path("/Users/test/financial-services")
+    check.ROOT = root
+
+    test_path = Path("/Users/test/financial-services/plugins/test/file.md")
+    result = check.rel(test_path)
+
+    assert result == "plugins/test/file.md"
+
+
+def test_err_function():
+    """Test the err() function appends to errors list."""
+    import check
+
+    check.errors = []
+    check.err("test error")
+
+    assert len(check.errors) == 1
+    assert check.errors[0] == "test error"