Facilitate session selection

[shikokuchuo]

Closes #14.

This PR implements:

1. Now the response to a list_r_sessions tool call shows the session number i instead of the sessions just being a numbered list. This makes it much easier to select a session by number.
2. I've made discovery default to at least 8 sessions (we always attempt to dial 1 - 8) so that they can really come and go. I think this is probably enough, but it's not very costly to dial a few additional IPC socket addresses if we wanted to.
3. As describe_session() only generates text, we can also use raw mode sends for that query (avoiding R serialization).

@simonpcouch feel free to optimize the prompt if you see a way to do this! Another FYI @wch.

[simonpcouch]

Nice!!!

Here's a log of a chat where the listing works as I'd expect: https://claude.ai/share/b2a7e33c-5c18-4edd-a2a1-8052e5fb32f0

That said, in the last step where the tool calls returns only indices 1 and 3, it lists them out as 1 and 2 in its user turn. Will try and adjust the prompt to emphasize that the indices should be preserved even if they skip over some integers.