Request a PTY for remote exec so build output streams live

Without a PTY, ninja/gclient/python block-buffer their output when stdout
isn't a terminal, which can make a multi-hour build look stalled in the CI
log for long stretches even though it's progressing fine.

main.go

@@ -415,6 +415,25 @@ func sshSession(st *state) (*ssh.Session, *ssh.Client, error) {
 		client.Close()
 		return nil, nil, err
 	}
+
+	// A PTY makes the remote side (ninja, gclient, python) think it's
+	// talking to a terminal, so it line-buffers instead of block-buffering
+	// for a non-tty pipe -- without this, output for a multi-hour build can
+	// arrive in laggy multi-KB chunks instead of streaming as it happens.
+	// Combines stderr into stdout, which is fine for a CI log (one
+	// interleaved stream is what you'd see watching a real terminal
+	// anyway).
+	modes := ssh.TerminalModes{
+		ssh.ECHO:          0,
+		ssh.TTY_OP_ISPEED: 14400,
+		ssh.TTY_OP_OSPEED: 14400,
+	}
+	if err := session.RequestPty("xterm", 40, 200, modes); err != nil {
+		session.Close()
+		client.Close()
+		return nil, nil, fmt.Errorf("requesting pty: %w", err)
+	}
+
 	return session, client, nil
 }