[Agent][QA/Integration] Gate real-account readiness and release safety

[karnaksp]

Objective

Keep end-to-end validation strong enough to distinguish sample readiness from real-account readiness.

Acceptance

• make acceptance-local passes for sample metadata.
• make acceptance-real passes only when a real token is configured and latest manifest source is yandex_music.
• make real-gate-smoke rejects sample metadata for real readiness.
• Release and Pages workflows never require or expose a private token.