When using SSH (ssh coder.workspace), the connection remains active after being disconnected from agent.

Steps to reproduce:

1. coder create workspace
2. ssh coder.workspace
3. coder delete workspace (or stop, start)

After step 3, the SSH command remains running until user input. After the switch to Tailscale networking, this applies for both ssh using ProxyCommand and coder ssh. (Also applies to other commands using SSH, like sftp.)

Typically when an SSH server goes away, the client is notified and we see something like:

Connection to [host] closed by remote host.
Connection to [host] closed.

Ideally we would also detect this client-side by checking workspace status from the coder API, if we're trying to connect to an outdated agent (i.e. workspace state changed after connection), we should either:

1. Disconnect and exit
2. Reconnect with a "Reconnected, but previous state was lost" (this is not a good message, just to illustrate)