[FEATURE]: Create and customize Agents from OpenCode Desktop

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I would like OpenCode Desktop to provide a graphical interface for creating and customizing Agents, similar to an Agent builder.

Currently, creating or changing a custom Agent requires using the CLI or editing configuration files manually. It would be much easier if users could do this directly from the Desktop app.

The Agent editor could allow users to configure:

• Name and description
• System prompt or instructions
• Model
• Agent mode
• Available tools
• Skills
• MCP servers
• Permissions
• Global or project-specific scope

It would also be useful to duplicate, edit, delete, enable, or disable an Agent and test it in a small preview chat before using it in a real session.

The GUI should use the same standard Agent configuration already supported by OpenCode, rather than creating a separate Desktop-only format.