Digest, chunk-embed, and question-embed keys include fleet|restricted so
a fleet caller cannot observe a restricted cache hit. Digest.Run takes
the class from permit_restricted.
A permit_restricted caller whose keys.toml workspace is not
guardian-star is refused at Search (no Honcho request) and flagged at
serve start and keys reload.
docs: P3 restricted callers search like the fleet; cache/logs isolate.
kyle
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