Fix: Remove unused mypy ignores and fix type errors

[tleonhardt]

This PR removes unused mypy ignores and fixes type errors.

Also:

• Fix a typo in comment in argparse_completer.py
• Add draft GEMINI.md file for use with Gemini CLI

About half the work in this PR was done by the Gemini CLI, including the first commit and PR creation.

I needed to finish about half of the unused ignores manually.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (8bbe688) to head (793c22e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1481   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files          23       23           
  Lines        4883     4884    +1     
=======================================
+ Hits         4822     4823    +1     
  Misses         61       61           

Flag	Coverage Δ
unittests	98.75% <100.00%> (+<0.01%)	⬆️

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