When the daemon exits before `hello`, the error has only the exit code.
For example, with `loginShell: true` and a profile that aborts on a
missing file:
```
pi-env exited with code 3 before it was ready
```
`sshConnection`'s probe and deploy steps already report ssh's stderr, so
this is about the launch itself: a login profile, a daemon that can't
execute, or a `Connection` created with its own command, which has no
probe or deploy step, so auth and network errors also end up here.
This keeps a short stderr tail until `hello` and adds it to startup
errors:
```
pi-env exited with code 3 before it was ready
remote stderr:
profile: /opt/toolchain/env.sh: No such file or directory
```
The tail is capped at 20 lines / 2000 chars, with terminal escapes and
control characters stripped. Ready sessions are unchanged.
I hit it while testing `loginShell` against an sshd in Docker. Tests are
in `test/connection.test.ts`, and `npm run check` and `./test.sh` pass.
