Add release candidate safety checker

[rotatedcoded]

Adds a local release candidate safety checker.

• Checks required runtime/source files
• Checks Stockfish binary availability
• Blocks staged generated data
• Blocks tracked generated data, except allowed .gitkeep placeholders
• Reports dirty source files before release
• Prints recommended validation commands

Validation:

• python -m pytest tests/test_release_candidate_check.py passed
• python -m pytest tests/test_runtime_readiness.py passed
• python -m pytest tests/test_ui_unified_persona_pipeline.py passed
• python ./check_release_candidate.py --engine-path engines/stockfish/stockfish.exe --allow-dirty-code passed