Migrate toplevel tool configs to pyproject.toml

[rolandwalker]

Description

• migrate tox.ini to pyproject.toml sections
• migrate pytest.ini to pyproject.toml sections

Motivation: tidy the toplevel directory.

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. Regression: pytest config section is invalid in pyproject.toml
   In pyproject.toml, the PR adds [tool.pytest], but pytest reads ini-style options from [tool.pytest.ini_options].
   Impact: addopts = ['--ignore=mycli/packages/paramiko_stub/__init__.py'] will likely be ignored, so test discovery/collection behavior changes from current pytest.ini behavior.
   Fix: rename section to:

   [tool.pytest.ini_options]
   addopts = ["--ignore=mycli/packages/paramiko_stub/__init__.py"]

No other correctness/security issues stood out in this PR.
Residual risk: I couldn’t run pytest locally in this environment (pytest not installed), so this is static review only.