codeSplitters feed: should the bring-your-own digest stay admin-only on the hosted site? #575

#538 (for #528) gates digest_feed on the hosted site the same way as coding agents: with AGENT_SANDBOX set, anyone not in ADMINS gets a 403. So ordinary hosted users get the raw feed but can't run the digest. #528 didn't scope it that way.

This needs a product decision: keep it admin-only (cost control, same as agents), or open it with a rate limit or per-user quota. If it opens, add a rule in ruleFor/RULES and a test for a non-admin caller.

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