`coder ssh` should exit when its parent process dies

Problem:

We observed an internal user having a large number of orphaned coder ssh processes running on their laptop.
This was contributing to increased load on coderd.
Our hypothesis is that the user's editor (Cursor) was automatically running a setup script over SSH that invoked coder ssh (via ProxyCommand in .ssh/config) and exiting with an error.

Steps to reproduce:

1. Run a new shell and get its PID

bash
$ echo $$
12345

1. SSH into a workspace and run a long-lived command

coder ssh my-workspace sleep infinity

1. Validate that the coder ssh command is running

ps -eo pid,ppid,command | grep [c]oder
12346 12345 coder ssh my-workspace sleep infinity

1. Kill the SSH process' parent (the bash shell)

kill -9 12345

1. Observe the coder ssh process still running but now owned by PID 1

ps -eo pid,ppid,command | grep [c]oder
12346 1 coder ssh my-workspace sleep infinity