Update agent docs away from split state/district H5 datasets

PolicyEngine/populace#237 removed release generation for one-file-per-state and one-file-per-district artifacts. The replacement path is the single certified Populace national H5 plus the standard region/scoping machinery in policyengine.py, not direct hf://policyengine/policyengine-us-data/states/... or districts/... files.

GitHub code search still finds split-H5 references in this repo:

• skills/tools-and-apis/policyengine-microsimulation-skill/SKILL.md
• skills/analysis/policyengine-district-analysis-skill/SKILL.md
• skills/domain-knowledge/policyengine-healthcare-skill/SKILL.md

Migration checklist:

• Stop constructing or documenting policyengine-us-data/states/*.h5 and policyengine-us-data/districts/*.h5 dataset URIs.
• Use the certified Populace default dataset through policyengine.py, with region filters/scoping for state and congressional-district analyses.
• Add or update a regression test so future state/CD runs do not depend on split regional H5 files.
• Keep historical/generated output unchanged unless it is actively used by the app or docs.

Read the three named SKILL.md files and the standard region/scoping machinery in policyengine.py, then search the repository for split state and district H5 URIs. Update the documented paths to use the certified Populace default dataset, add or update regression coverage for state and congressional-district runs, and verify that no active docs or app paths depend on split regional H5 files.