only show STDERR with `-v` or `-debug`

[durandom]

This is super helpful for debugging, but if an agent executes mcp-cli and knows what it's doing it always adds a lot of noise and eats tokens.

[claytono]

I'd like to second this. Some MCPs like https://github.com/sirkirby/unifi-network-mcp produce dozens of lines of output when on start up. I'd be nice to have an environment variable or config option to turn this off unless the MCP exits non-zero.