[0.5.0][Stretch] Audit export hooks (webhook/file sink)

**Stretch goal (pull in if the quarter allows).**

### Goal
Get policyAudit + auth events out to an external sink.

### Scope
- Webhook/file export hooks for audit events. A full queryable/retained audit store is deferred to a later release.

### Key files
`runtimes/typescript/src/lib/audit.ts`, config schema

**Depends on:** none
