- Update deprecated frequency strings to lowercase (H→h, S→s)
- Add explicit compat and join parameters to xarray merge to avoid FutureWarnings
- Apply ruff formatting
- Add Git workflow guidelines to CLAUDE.md
Changes:
- src/modelskill/comparison/_comparison.py: Add explicit compat="no_conflicts" and join="outer" to merge()
- src/modelskill/model/dummy.py: Change freq="H" to freq="h" in docstring example
- tests/test_simple_compare.py: Change freq="4S" to freq="4s"
- CLAUDE.md: Add Git Workflow section with reminder to never commit directly to main
- Format code with ruff
All tests pass (589 passed, 5 skipped) with only expected UserWarnings remaining.

Pandas 3.0 changes default datetime precision from datetime64[ns] to
datetime64[us], which causes xarray.interp() to fail when source and
target datasets have mismatched precision. Additionally, pd.cut() NaN
handling and StringDtype compatibility changed.
Changes:
- Normalize datetime coordinates to datetime64[ns] for backward
 compatibility with pandas 2.x in _convert_to_dataset() and
 _parse_dataset()
- Fix pd.cut() NaN handling: check pd.notna(x) before accessing .mid
- Replace np.issubdtype() with pd.api.types.is_numeric_dtype() for
 pandas 3.0 StringDtype compatibility
- Fix peak_ratio metric: use .total_seconds() instead of hardcoded
 nanosecond division to work with any datetime precision
- Replace peak_ratio test with synthetic data test that has verifiable
 peaks (ratio=1.1)
- Improve code clarity: iterate over self.data instead of matched_data,
 use .get("kind") to handle auxiliary variables safely
- Remove unused imports in test files (ruff auto-fix)

Test both pandas versions on every PR to ensure compatibility
with both versions until pandas 2.x support is dropped.

Reverted changes that were not related to Pandas 3 compatibility:
- CLAUDE.md git workflow docs
- _comparer_plotter.py formatting
- _comparison.py raw_mod_data refactoring and merge params
- Test file formatting and unused import cleanup
These changes are tracked in issues #568 and #569 for separate PRs.