ci: add dependency health checks

[rian-be]

Summary

This PR adds repeatable dependency health checks to the CI pipeline for ModularityKit.Mutator and documents the same workflow for local use.

Highlights

CI workflow

• Add a dedicated dependency-health job to pr-check.yml.
• Run dotnet list package --vulnerable --include-transitive for security coverage.
• Run dotnet list package --outdated --include-transitive for package freshness.
• Keep the dependency check separate from publishing and release automation.

Local workflow

• Add a small helper script at scripts/dependencies/check_package_health.py.
• Make the same check runnable locally after restore.
• Document the command and remediation path in README.md.
• Ignore Python bytecode artifacts with __pycache__/ and *.pyc.

Why

The repository has multiple package surfaces across runtime, governance, Redis, examples, benchmarks, and tests. A shared dependency check keeps vulnerability and outdated-package signal visible in CI without requiring manual inspection across projects.

The helper script keeps the local command aligned with CI and makes the output easier to act on while still relying on native dotnet list package behavior.

Checks

• python3 -m py_compile scripts/dependencies/check_package_health.py
• python3 -m scripts.dependencies.check_package_health --help
• git diff --check

Closes

• Closes [Task]: Add dependency and security checks to CI #54