Add harbor vulnerability dashboard command to view CVE vulnerability status

## Problem Statement

Harbor provides vulnerability scanning via `harbor scan-all` and CVE allowlist management via `harbor cve-allowlist`, but there is no CLI command to view an overall vulnerability dashboard.

Users currently cannot:
- View a summary of vulnerabilities across all projects
- Identify images with critical/high severity CVEs
- Get a high-level security posture from the CLI

This limits the CLI’s usefulness for security monitoring and forces users to rely on the WebUI for basic visibility.

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## Proposed Solution

Introduce a new `harbor vulnerability` command that provides a dashboard-style view of vulnerabilities across the Harbor instance.