pre-populate the registry URL where it makes sense ( in TUI, the arctl agent [add-mcp/add-skill/add-prompt] commands) #145
Description
For the commands that take in the registry URL value, we should make those flags optional and then just use whatever the default registry URL is (checking the environment variable).
This also applies for the agent add-mcp command, in the TUI where we ask the user to enter the URL.