Reliable multi-tenant auto setup audit capability

## Development Mock Backend

The first implementation is intentionally tiny: a replaceable audit interface
with a mock file backend.

By default it writes JSONL audit events to:

```text
/tmp/audit-core/audit-YYYYMMDDTHH.jsonl
```

Files older than 7 days are removed when the backend writes or when cleanup is
run explicitly. This backend is for local integration and bootstrap wiring. It
is not durable audit custody.

Example:

```bash
python3 -m audit_core emit \
  --source openbao \
  --action openbao.authenticated_readiness_proof \
  --resource openbao/openbao-0 \
  --outcome success \
  --detail file_audit_visible=true \
  --detail backend=mock-file
```

Cleanup:

```bash
python3 -m audit_core cleanup
```

Make targets:

```bash
make test
make mock-audit-smoke
make mock-audit-cleanup
```

Environment:

- `AUDIT_CORE_MOCK_DIR`: override the output directory.
- `AUDIT_CORE_MOCK_RETENTION_DAYS`: override the default 7-day cleanup window.