Add a persisted cross-invocation cache of resolved policy verdicts

[JSONbored]

Problem: Even with conditional-GET caching (above), the resolved AI-usage-policy verdict for a repo is recomputed fresh on every invocation rather than persisted across runs.

Area: AMS / Performance

Proposal: Persist the resolved policy verdict per repo locally, invalidated when the underlying doc's ETag changes.

Deliverables:

• A small local cache of resolved policy verdicts, keyed by repo and doc ETag.

Acceptance criteria:

• Repeated runs against the same unchanged repo skip re-resolution entirely.

[linear-code]

JSO-3503