test: isolate temporary skill registry state

[AlanAAG]

Summary

test_load_from_tmp_dir calls load_skills(tmp_path), which intentionally
replaces the module-level skill registry. The temporary registry then leaks
into later tests, leaving the default feynman skill unavailable and causing
two order-dependent failures in test_skills_default.py.

This change uses pytest's monkeypatch fixture to restore the registry after
the temporary-directory test. Production behavior is unchanged.

Verification

• uv run pytest -q tests/test_skills.py tests/test_skills_default.py
  • 33 passed
• uv run pytest -q
  • 6 failed, 545 passed, 23 skipped

The six remaining failures are pre-existing feature specifications in
tests/test_product_memory.py.