Utility to switch Fly.io accounts

A utility (perhaps via direnv) to maintain and switch between multiple Fly.io accounts would be a time saver. Currently, the process involves logging in and out of Fly.io via `flyctl auth login` and `flyctl auth logout`.

References:
[Feature request: easier toggling between accounts using flyctl](https://community.fly.io/t/feature-request-easier-toggling-between-accounts-using-flyctl/1641)

No file, test, or entry point is named. Start by reviewing the repository's shell configuration and the referenced Fly.io account-switching request, then compare the current flyctl auth login and logout flow with the desired multi-account behavior. Done should mean a documented, usable way to switch accounts without repeatedly logging in and out.

Written by the indexing model from the issue text.