fix: make paper qualification runtime explicit

[stefan-jansen]

Closes #60

Summary

• create a clean Python 3.12 environment for paper qualification
• pin and install the qualification script's psutil runtime dependency
• use the same explicit interpreter for artifact binding and candidate installation
• enforce the runtime contract in workflow policy tests

Verification

• uv run pytest tests/unit/test_workflow_policy.py -q
• uv run python scripts/qualification/check_workflows.py
• uv run ruff check --no-fix src tests scripts
• uv run ruff format --check src tests scripts
• uv run ty check
• uv run pytest
• uv run mkdocs build --strict
• uv build
• pre-commit run --all-files

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