ci: clear Node 20 deprecation warnings on self-test workflow

[d33pm3]

Why

main CI was already green. The self-test job is a real baseline (pip install -r requirements.txt, py_compile, version pin, regression, smoke), not an empty pass.

The only proven defect in the latest main logs was a first-party Actions runtime warning:

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-python@v5.

This is a Python skill/pipeline repo. There is no package.json. npm/npm ci was not added.

Change

• actions/checkout@v4 → @v5 (Node 24), persist-credentials: false
• actions/setup-python@v5 → @v6 (Node 24)
• permissions: contents: read
• workflow_dispatch so the same job can be re-run on a branch

Evidence

• Before (main): https://github.com/d33pm3/Policy-Extract/actions/runs/34032045127 — success with Node 20 warning
• After (this branch): https://github.com/d33pm3/Policy-Extract/actions/runs/34076374572 — success; compile/regression/smoke still run

Out of scope

• App / skill / script logic
• Dependency ranges in requirements.txt / pyproject.toml
• Secrets / LLM integrations
• Merge to main (needs explicit approval)