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()

ROADMAP.md

@@ -44,10 +44,10 @@ Additional functionality:
 
 - [ ] `labels` command - manage artifact labels
 - [ ] `copy` command - copy artifact between repos
-- [ ] `gc` command - show garbage collection status
+- [x] `gc` command - show garbage collection status
 - [ ] `replication` command - list replication rules/executions
 - [ ] `quota` command - show project quota usage
-- [ ] `audit` command - show audit logs
+- [x] `audit` command - show audit logs
 - [ ] Partial digest tab completion
 
 ## Phase 5: Distribution
@@ -59,11 +59,29 @@ Packaging and distribution:
 - [ ] Man page generation
 - [ ] Shell completions (bash/zsh/fish)
 
+## Phase 6: Admin (Complete)
+
+Admin capabilities for Harbor system management:
+
+- [x] `system` command - version, auth mode, storage volumes
+- [x] `gc` command - schedule, history, manual trigger
+- [x] `gc-run` command - trigger GC with dry-run/wait
+- [x] `users` command - list users with pagination
+- [x] `user-create` command - create local user
+- [x] `user-delete` command - delete user
+- [x] `user-admin` command - grant/revoke admin privileges
+- [x] `user-passwd` command - reset user password
+- [x] `audit` command - view audit logs with filters
+- [x] `audit-purge` command - purge old audit logs
+- [x] `cve-allow` command - manage system CVE allowlist
+- [x] `scan-all` command - trigger system-wide vulnerability scan
+
 ## Dependencies
 
 ```
 Phase 1 (done) ──> Phase 2 ─┬─> Phase 3
-                            └─> Phase 4 ──> Phase 5
+                            ├─> Phase 4 ──> Phase 5
+                            └─> Phase 6 (done)
 ```
 
-Phase 3 and 4 can proceed in parallel after Phase 2.
+Phase 3, 4, and 6 can proceed in parallel after Phase 2.