feat(cli): forward stdin and Ctrl-C in ax ssh

[anzal1]

Part of #374

ax ssh <task> with no command starts /bin/sh in the sandbox, but guest.Client.Exec never opened the process's stdin. The shell read EOF right away and exited, so the command returned without printing anything.

The guest protocol in agent-substrate/env already covers this: StartProcessRequest.stdin opens a stdin pipe, WriteProcessInput streams into it, and SignalProcess sends a signal to the process group. This PR wires the client up to those calls.

Changes

• guest.ExecOptions has two new fields:
  • Stdin io.Reader is streamed to the process through WriteProcessInput. Remote stdin is closed when the reader hits EOF. A nil Stdin keeps the old behaviour, where the process gets an empty stdin.
  • Signals <-chan os.Signal sends each SIGHUP, SIGINT, SIGQUIT or SIGTERM it receives to the process group through SignalProcess.
• ax ssh passes os.Stdin and forwards SIGINT. Ctrl-C now interrupts the remote command instead of killing ax and leaving the remote process running.
• When stdin is a terminal, the default shell runs as /bin/sh -i. With -i it prints a prompt, and when Ctrl-C reaches the process group it stops the foreground command but not the shell.

So ax ssh task123 gives you a working shell: you get a prompt, commands run, Ctrl-C interrupts, Ctrl-D or exit leaves, and the exit status is passed back. Piping also works now, for example cat script.py | ax ssh task123 -- python3 -.

What's still missing

The guest protocol has no PTY. Input is line-buffered by the local terminal, and the remote side has no tty. That means full-screen programs like vim, top and less, job control, and window resizing don't work yet. Supporting them needs a change in agent-substrate/env: a PTY option on StartProcessRequest and a way to send resizes. I can open an issue there if that sounds right. Once it exists, ax needs to put the local terminal into raw mode and send SIGWINCH sizes, which is a small follow-up.

Testing

internal/guest/client_test.go is new. It runs the real guest server from agent-substrate/env on a local port and covers:

• stdin bytes reach the process and its output comes back
• line-by-line interactive input over a pipe, where each line is sent only after the reply to the previous one arrives, and closing stdin gives the process EOF
• the exit code is passed through
• with no Stdin, the process still gets an empty stdin
• a forwarded SIGINT ends the process with 130

Four of these fail on main. The interactive test on main prints just bye, which is the bug in this issue: the loop sees EOF straight away.

I also checked /bin/sh -i by hand inside Linux containers: dash (the default python:3.12-slim task image), bash on Ubuntu 22.04, and busybox ash on Alpine. In each one, Ctrl-C during sleep 30 stopped the sleep within a second, and the shell kept accepting commands.

go test -race ./..., go vet ./... and make build pass, and go mod tidy leaves go.mod and go.sum unchanged.

Notes

• ax ssh now forwards stdin for one-off commands as well, which matches what ssh does. So a script that runs ax ssh inside a while read loop will have the loop's input read by the remote command. If you'd rather only forward stdin for the interactive shell, or add an -n flag like ssh has, I'm happy to change it.
• Bash has no tty here, so it echoes some readline redraw sequences. That goes away once there's a PTY.
• The guest kills processes after its one hour default timeout, and interactive sessions are no exception.