Add runtime readiness checker

[rotatedcoded]

Adds a runtime readiness checker for UI play.

• Checks persona file availability
• Checks Stockfish path
• Checks Direct Ranker model path
• Checks elite guarded evidence status
• Reports recommended UI move policy
• Returns exit code 0 when ready and 2 when blocked/missing
• Supports --strict-metrics for blocking metrics checks

Validation:

• python -m pytest tests/test_runtime_readiness.py passed
• python -m pytest passed