Support cross-host R sessions with sess

The issue mentions the new 'sess' architecture using bidirectional JSON-RPC over a stream socket. Start by examining the sess implementation in the codebase, likely in the src/ directory, to understand the current transport mechanism. Look for socket handling and connection logic. The goal is to modify sess to accept connections from a different host, possibly via TCP. Testing will involve setting up a cross-host scenario, perhaps with SSH forwarding, to verify the session attachment works.

There have been repeated requests to attach vscode-R to an R session running on a different host from the VS Code extension host, particularly in HPC environments where VS Code runs on a login node while R runs on a scheduler-allocated compute node.

Related issues include #469, #1019, #1047, #1276, #1359, #1391, #1506, and #1541.

Historically this was difficult because the session watcher depended on shared files, file watchers, and local process IDs.

With the new sess architecture, communication is now bidirectional JSON-RPC over a stream socket.
This should make cross-host sessions feasible in the future, for example through SSH socket forwarding or an additional TCP transport, without changing the higher-level session protocol.

I don't think vscode-R needs to manage SSH, Slurm, or other schedulers itself.
The useful boundary would be simply allowing sess to connect when the R process and the extension host are on different machines.