mason: stabilize release diagnostics and FM-OC-6

Route Pi doctor failures through PromptIO so repair tests capture their expected pre-repair FAIL diagnostics instead of leaking them into the release log.

Correlate FM-OC-6's overflow response to the arm prompt and give provider arrival its own readiness stage, removing the unrelated system-banner assumption without increasing any timeout budget.

Co-Authored-By: Alfonso <alfonso@cortexkit.io>