Test/repo housekeeping: stale lint reference, orphaned fixtures, hardcoded local path

[bdestombe]

Small cleanups found while building out the test suite. None affect model results.

1. Stale lint target — pyproject.toml:85 lints tests/test_pwnlayers.py, deleted in b86583e. The lintformat:lint script fails or silently skips depending on ruff's version.
2. Orphaned test fixtures — tests/data/ is git-tracked but referenced by no test (only a licence file and a README stating the values are fake).
3. Untracked leftovers — tests/model_ws/ holds ~51 MB of gridgen output from Dec 2024, referenced by no test and gitignored. Safe to delete locally.
4. Empty test shells — tests/test_nhflo_utils.py (0 bytes) and tests/test_hhnk.py (a lone docstring) collect nothing while suggesting nhflo_utils.py (23 functions) and hhnk.py (4 functions) are covered.
5. Hardcoded developer path — geoconverter/geoconverter.py:279 embeds /Users/bdestombe/...; it should derive the location from nhflodata.get_paths.
6. Stale local hatch env — the registered test env is Python 3.11 and can no longer resolve dependencies (the project requires >=3.14); it needs hatch env remove test once per developer machine.

Items 1 and 4 are addressed in the test-suite PR; the rest are left here so they are not lost.