fix(scanner): clean dead-code warnings

[duriantaco]

Summary

• remove genuinely unused pickle static scan symbols reported by Skylos
• replace dynamic eval-config AST visitor hooks with explicit ast.walk checks so Skylos no longer flags them as unused
• add regression coverage for Python assignment, annotated assignment, and dict-literal eval config scanning

Verification

• python -m pytest -q
• python -m ruff check .
• python -m compileall -q ceres tests scripts
• skylos . --quality --no-upload --file-filter ceres/analyzers/model/pickle_static.py --format concise (no dead-code findings remain for this file; other quality findings still reported)
• skylos . --quality --no-upload --file-filter ceres/analyzers/eval/safety_config.py --format concise (no dead-code findings remain for this file; other quality findings still reported)