feat(server): visibility-aware `kb.audit` queries

[plind-junior]

What you're trying to do

gbrain's audit is per-user visibility-filtered (the "company brain" pattern). Vouch's kb.audit returns every event; once #2 (multi-dim scopes) lands, a reviewer with project-scope-A should not see project-scope-B's audit events. Bake the filter into the audit reader so the visibility model is enforced by the read path, not by the caller.

Suggested shape

• kb.audit accepts a viewer_scope: ScopeSpec param (passed by the transport's auth layer).
• audit.read_events filters events whose object_ids reference artifacts outside viewer_scope.
• Fallback: events with no object_ids (e.g. kb.init) are visible to everyone.

Acceptance

• A two-project KB with reviewer A in project-A sees no project-B events.
• A fuzz test asserts no project leak across viewers (mirrors gbrain's "zero leaks" claim).

Out of scope

• Encrypted audit log entries (visibility is filtering; encryption is a separate ask).
• Cross-viewer aggregate queries.