build(infra): add contrib package discovery script

[lan17]

Summary

• add a stdlib-only scripts/contrib_packages.py helper with list, names, matrix, and verify commands
• discover real contrib packages from evaluators/contrib/*/pyproject.toml while excluding template
• add budget and cisco to root semantic-release version_toml

Why

This centralizes contrib package discovery for later Makefile, CI, and release work and makes wiring drift explicit when a contrib package is missing from root packaging metadata.

Validation

• python3 scripts/contrib_packages.py list
• python3 scripts/contrib_packages.py names
• python3 scripts/contrib_packages.py matrix
• python3 scripts/contrib_packages.py verify (currently fails on main because evaluators/builtin/pyproject.toml is still missing the budget extra and uv source wiring, which this PR intentionally does not edit)
• temporary failure-path check: removed the cisco root version_toml entry locally, confirmed verify reported it, then restored the file
• python3 -m py_compile scripts/contrib_packages.py

Impact

Later automation can consume names for shell iteration and matrix for JSON-driven CI/release fan-out, while verify remains the drift gate for root wiring.

Design Context

• Shared design doc and implementation plan: https://gist.github.com/lan17/823617097f4b7e9f4ff717979de7dba4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[lan17]

Superseded by #193, which rolls up this reviewed stack into one PR while preserving the logical commit boundaries for review.