Clean up dotfile created by test runs

[rolandwalker]

Description

Running the full test suite via tox seems to create a .myclirc file in the repo root, which can cause trouble later, including failures in subsequent test runs. Add a tox cleanup step to remove the dotfile.

It's not clear where or how this file gets created, but it is easy enough to unconditionally delete it.

The feature of reading from a .myclirc file in the current working directory is also questionable! Perhaps a warning should always be shown.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Potential data-loss regression for local repo config
   commands_post = rm -f -- ./.myclirc will always delete a project-local .myclirc after tox runs, even if it existed before tests. This can remove a developer’s intentional repo-scoped config (which mycli explicitly supports via pwd_config_file).
   File: tox.ini
   Action: only remove files created by the test run (e.g., track pre-existence in commands_pre/post logic), or isolate tests so they never write to repo root.

2. Portability regression in test runner cleanup
   Using external rm in tox post-commands is Unix-specific and will fail on environments without rm (notably Windows).
   Files: tox.ini, tox.ini
   Action: replace with a cross-platform Python cleanup command (for example python -c "from pathlib import Path; Path('.myclirc').unlink(missing_ok=True)") and allowlist python if needed.

No security-specific issues found in this PR.