Avoid using pexpect in poetry shell

poetry shell has a surprising behavior of spawning a pexpect wrapper which proxies I/O streams between the user and the application. This makes poetry shell unsuitable as a general purpose shell configuration tool: aside from the performance implications, pexpect does not deal correctly with programs that directly access the tty to interact with the terminal (and was never intended to be a full blown terminal application proxy).

It would be much better if poetry shell could simply spawn the shell via os.exec() after configuring the environment, to avoid having to proxy anything. This would be more consistent with user expectations and would prevent applications from breaking when running inside poetry shell.

Start at the poetry shell entry point and trace how the pexpect wrapper configures the environment and proxies I/O. Replace that process handoff with the requested os.exec() approach, then verify that terminal applications work without an intermediate pexpect proxy.