It's also not clear to me why using "skills" would consume less context once invoked.

It's just instructions with RAG. The more I read about this the more convinced I am that this is just marketing.

Skills wont use less context once invoked, the point is that MCP in particular frontloads a bunch of stuff into your context on the entire api surface area. So even if it doesn't invoke the mcp, it's costing you.

That's why it's common advice to turn off MCPs for tools you dont think are relevant to the task at hand.

The idea behind skills us that they're progressively unlocked: they only take up a short description in the context, relying on the agent to expand things if it feels it's relevant.