What you're describing is a harness implementation detail not something specific to the MCP protocol or even how skills behave. Both of those are effectively providing the same level of information to the harness. Harnesses have traditionally (and still generally do) exposed enabled MCP servers and their actions ahead-of-time in the system prompt directly instead of doing progressive disclosure.

You can't do that with tools either. Skills are basically prompts - they're not analogous to tools or MCPs. I'm not sure what your point is

you can actually invoke an MCP App by name in both Codex and Claude using the @ handle

Use an MCP proxy.