Set per-agent eval configuration

Body for PUT /v1/agents/{id}/eval-config.

Both fields are optional so the UI can flip one switch at a time without having to re-assert the other; a field left unset means "no change".

Per-agent eval configuration exposed by /v1/agents/{id}/eval-config.

Two independent toggles: evaluate_live gates SessionType.LIVE (production) sessions and evaluate_test gates SessionType.TEST (dev-initiated) sessions. Simulation runs always evaluate.