This change refactors the command line message composer to unify the
various composition methods (interactive, non-interactive, reply,
forward, and template-based) into a single function.
The detection of non-interactive sessions was also changed to use a more
reliable check on `os.Stdin` instead of inspecting arguments.
A message preview and confirmation prompt has been added to the
interactive mode, allowing the user to review or edit the message before
it's posted to the outbox.
