[Proposal] Add initial Agent & Principal visibility UI

[jiwlee97]

Is your feature request related to a problem? Please describe.
Currently, Argo CD Agent users can only inspect agent-related information through the argocd-agentctl CLI. This makes it difficult to understand the overall Principal-Agent relationship and to monitor connection status directly from the Argo CD web interface.

Describe the solution you'd like
For v1, I propose adding a lightweight "Agent & Principal Visibility UI" to Argo CD — focusing purely on connection overview and status visibility.

Initial Scope (v1):

• Add an “Agents” navigation item in the sidebar
• Provide Tile/List View for Principals and Agents
• Provide Tree View to visualize the Principal → Agent relationship
• Provide List View for quick inspection of all agents
• Provide Detail View for both Principal and Agent configuration

The main goal of v1 is visibility — allowing users to see how Principal and Agents are connected and operating, all within the Argo CD UI.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
For the initial version, the focus will remain simple — providing a basic overview of Agent and Principal information inside the Argo CD UI. Future versions (v2+) can gradually expand functionality.

[mikeshng]

Given that argocd-agent is an optional component to Argo CD, is this a new UI or existing Argo CD UI? Let me take a look at the PR.