docs: add audit use cases

docs/audit-use-cases.md

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+# Audit Use Cases
+
+RepoScope is useful when a team needs a quick, repeatable view of repository health before making a change or handing a project to another maintainer.
+
+A maintenance audit can check whether the README points to valid paths, whether expected workflow files exist, and whether the repository exposes clear setup instructions. A release-readiness audit can record the current diagnostics, unresolved findings, and the owner responsible for follow-up. A handoff audit can preserve a redacted snapshot of the checks that were run without sharing tokens or private source.
+
+Audit results should identify the repository and timestamp, distinguish observations from recommendations, and make rate-limit or authentication limitations visible. A score is a navigation aid, not a security guarantee or a substitute for reading the underlying findings.