Account rotation — credential adapter

Choose the credential tool from both host and agent family, perform only the explicit account switch, and report the identity observed by the matching runtime.

Verifying identity through the target runtime works because the runtime is the only party whose opinion matters: credential files can be swapped perfectly and still authenticate as the old account in an already-running process.

Named failure mode — stale-process identity: declaring the rotation done while every live session still holds the previous account's tokens in memory.

Anti-pattern: confirming a switch by diffing credential file bytes.