refer to R sessions of interest as "hosts"

[simonpcouch]

Currently, we refer to service hosted in the R session of interest as the "server". This is a bit confusing, as the proxy is itself an MCP server. Let's rename mcp_serve() to mcp_host() and refer to it as the "host" rather than the "server".

Related to discussions in #11.

[wch]

I agree, "host" makes sense. And I think "proxy" should be renamed to "server", since that part actually is the MCP server.

[shikokuchuo]

Yes, that should make it easier for people to understand if they're looking at documentation on how/what to connect to their MCP client.