fix(onboarding): scope the mirror-probe cache to the auth identity

The probe cache keyed entries by owner/repo alone, but probeRepoMirrored consults the active context's core — so after 'entire auth use' the mirror rung could serve the previous identity's cached answer (mirrored or missing) for the rest of the 15-minute TTL. Cache keys now carry an identity scope: the active context name, or a digest of ENTIRE_TOKEN for env-token sessions.

Trail-Finding: 019f6b46-0118-7dea-b2bb-4eb947d47fc5