Drop computed retirement helpers from CPS export

[MaxGhenis]

Summary

• drop pension_income and retirement_distributions after they are used as SSI asset-imputation predictors
• explicitly delete stale persisted H5 keys so append-style CPS saves cannot leave computed variables behind
• extend the add_tips regression to cover both in-memory save payloads and stale H5 key deletion

Why

Publication run usdata-gha26236821283-a1 failed in Modal stage 1_build_datasets because extended_cps_2024 exported policyengine-us computed variables: pension_income and retirement_distributions. These leaked in from PR #1097 after being added as construction-only predictors for SSI asset imputation.

Verification

• uv run ruff format policyengine_us_data/datasets/cps/cps.py tests/integration/test_cps_generation.py
• uv run ruff check policyengine_us_data/datasets/cps/cps.py tests/integration/test_cps_generation.py
• uv run pytest tests/integration/test_cps_generation.py::test_add_tips_derives_tipped_status_from_raw_cps -q
• uv run pytest tests/integration/test_cps_generation.py tests/unit/test_extended_cps.py -q
• uv run --with pyyaml python scripts/run_quality_guards.py
• /cycle read-only review: clean