implement server discovery

[simonpcouch]

In #11, @shikokuchuo implemented mcp_discover() and select_server(). We'll just need to "plug those in," likely by surfacing them both as ellmer tools to the model, allowing the user to ask:

• mcp_discover(): What hosts / R sessions do I have available?
• select_server(): In this R session, do...

The names of those functions will likely change with #13.

We'll also need to think through what fallback looks like here—if the selected host is shut down, should we interpret that as the model just having made the request to whatever host is "first" in mcp_discover(), perhaps with a message noting that that request went to a different host when we send the response?

[shikokuchuo]

I noticed that #18 doesn't get us all the way there, as in the scenario where you start with 3 sessions and session 2 exits:

At this point, querying again it will list session 1 and 3 in a list as 1. and 2.
If I then ask to choose 2, it will select i = 2 rather than session 3, which is showing as 2. in the list.

I have a fix and will put up a PR.