fix: replace Go PTY tests with txtar using creack/pty (#2682)
The TestInteractiveTTY test was flaky on macOS because testscript's
native ttyin/ttyout uses Go's setctty syscall which hangs due to
Go bug https://github.com/golang/go/issues/61779.
This replaces the Go-based PTY tests with txtar tests using the new
pty-run command, which uses github.com/creack/pty that works correctly
on both Linux and macOS.
Changes:
- Add pty-run testscript command using creack/pty
- Add Command interface for cleaner command organization
- Convert PTY tests to txtar format (pty_echo, pty_interactive)
- Remove old Go-based pty/pty_test.go
- Update README with pty-run documentation
