fix(auth): make writes visible to reads while credentials are injected

[yuyang27]

Auth.all() returns the injected snapshot (or the MIMOCODE_AUTH_CONTENT fallback) and never consults the file when one is present, but set/remove only wrote the file. So while either channel is active, a write landed on disk and every later read in that process kept returning the pre-write state: an OAuth token refresh, a key rotation or a logout looked like it succeeded and changed nothing until the process restarted.

The env channel had this defect before inject existed — a refresh inside a workspace child was invisible to that same child.

A mutation through this service is authoritative, so the snapshot now moves with the write. It is updated only after the write lands: moving it first would leave the in-memory credentials ahead of the file on a failed write, and the next process would silently run on the old ones. An embedding host that re-injects from the file on its own schedule converges on the same content.

Tests: set/remove visible to later reads with inject active, the same for the env channel, and a failed write leaving the snapshot untouched (flip-verified — moving the update before the write turns that one red).

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[yuyang27]

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