Add python-skill and improve simulation mechanics skill

[nwoodruff-co]

Summary

• New python-skill: Documents the uv + Python 3.13 standard for all PolicyEngine Python work — uv venv --python 3.13, uv pip install, uv run, and venv conflict resolution (VIRTUAL_ENV mismatch warning)
• Expanded policyengine-simulation-mechanics-skill: Fixes root causes of Claude spending 6+ minutes on trial-and-error before writing working code:
  • CRITICAL section: correct environment setup (uv run python script.py)
  • CRITICAL section: exact import paths with a "There is NO" list to prevent guessing non-existent modules
  • UK dataset section: exact EFRS disk paths, ensure_datasets pattern, confirmed household variables (tenure_type string values, domestic_energy_consumption)
  • Direct data analysis example: working end-to-end pattern for decile/groupby/percentile analysis without Aggregate
• Registers python-skill in country-models, analysis-tools, data-science, and complete plugins

Test plan

• Load simulation-mechanics-skill and verify imports are correct first try
• Load python-skill and verify uv venv setup guidance is followed
• Run an EFRS energy analysis script — should work without exploration loops