Implement Phase 6 LoRA artifact validation

[sarvesh1327]

• enforce the Phase 6 real MLX LoRA artifact contract before candidate checkpoint registration
• link adapter, checkpoint, backend, and training manifests through artifact paths
• add backend manifest provenance for trajectory ids, profile id, base model, target modules, and LoRA rank

Verification

• python -m pytest tests/agent/advisor/test_real_training_backend.py tests/agent/advisor/test_training_runtime.py -q
• ruff check .
• python -m pytest tests/agent/advisor -q