feature: Standalone policy-digester unit (read → digest → store, serve digest to the flow) #2555

Part of the Digested access-control policy epic (#2537). Split out of #2539 (Policy Digester) during design grilling.

Background

Digestion happens once, out of band. #2539 delivers the pure digest_policy(source_policy: str) -> str conversion callable but does no I/O, persistence, or wiring. This issue is the runtime unit that uses that callable.

Goal

A standalone policy-digester unit that reads the original source policy, digests it, stores the digest, and arranges for the rest of the system to run on the digested policy — so onboarding, policy-update, and the PRB only ever see the already-digested policy via FilePolicySource (unchanged).

Scope (to be refined)

• Trigger model: when digestion runs — on source-policy change, manual invocation, or scheduled.
• Storage & serving: where the digest is stored and how FilePolicySource (AIAC_POLICY_FILE) is pointed at it; single-file vs multiple sources, and any concurrency concerns.
• Flow swap: cut onboarding/policy-update/PRB over from raw source to the stored digest. Consider whether the PRB proposer prompt needs retuning for digest-language input — it is currently framed for raw source prose (integration concern surfaced during feature: Source→digested policy conversion (Policy Digester) #2539 grilling).
• Conflict gate: integrate feature: Digest-layer authoring conflict detection (report + not-applied) #2554 — a digest with an authoring-layer conflict is reported and not stored.

Dependencies

• feature: Source→digested policy conversion (Policy Digester) #2539 (the digest_policy() conversion callable)
• feature: Digest-layer authoring conflict detection (report + not-applied) #2554 (digest-layer authoring conflict detection — the storage gate)

Acceptance criteria (draft)

• The unit reads the source policy, calls digest_policy(), and stores the digested policy.
• Onboarding / policy-update / PRB operate on the digested policy via FilePolicySource, with no change to their own code beyond what points the source at the digest.
• A digest that fails the feature: Digest-layer authoring conflict detection (report + not-applied) #2554 conflict check is reported and not stored.