add admin commands for system, users, gc, audit, security

Phase 6 implementation:
- system: version, auth mode, storage volumes
- gc, gc-run: schedule, history, manual trigger
- users, user-create, user-delete, user-admin, user-passwd
- audit, audit-purge: view and purge audit logs
- cve-allow: manage system CVE allowlist
- scan-all: trigger system-wide vulnerability scan

Helpers: paginated_request(), confirm_action(), format_size()

PROJECT.md

@@ -14,13 +14,18 @@ Command-line interface for managing Harbor container registry. Provides quick ac
 - Tag management
 - Artifact deletion
 - Project configuration (auto-scan, auto-sbom)
+- System information and storage status
+- Garbage collection (status, trigger)
+- User management (list, create, delete, admin privileges, password reset)
+- Audit log viewing and purging
+- CVE allowlist management
+- System-wide vulnerability scanning
 
 ### Out of Scope
 
-- User/group management (admin-only, rare)
-- System configuration (one-time setup)
+- LDAP/OIDC group management (external provider)
 - Replication rule creation (complex, rare)
-- Garbage collection triggers (scheduled)
+- Registry endpoint management (rare setup task)
 
 ## Success Criteria