ci: clear Node 20 deprecation warnings on RegExtractor workflow

[d33pm3]

Why

main CI was already green. The job is a real baseline (pip install -r requirements.lock, pytest collect + pytest, scripts/make_release.py), 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, actions/upload-artifact@v4.

This is a Python package. There is no package.json. npm/npm ci was not added. Python 3.11 and the lockfile were left unchanged. CI does not call an LLM.

Change

• actions/checkout@v4 → @v5, persist-credentials: false
• actions/setup-python@v5 → @v6
• actions/upload-artifact@v4 → @v5
• add pull_request trigger so PRs get the same job

Evidence

• Before (main): https://github.com/d33pm3/Reg-Extract/actions/runs/34034401765 — success with Node 20 warning
• After (this branch): https://github.com/d33pm3/Reg-Extract/actions/runs/34076994911 — success

Out of scope

• App / package / test logic
• Dependency ranges / lockfile contents
• Secrets / LLM integrations
• Merge to main (needs explicit approval)