chore(tooling): run codespell on all files; improve skip config

[danceratopz]

🗒️ Description

This PR extends spellchecking to all sources files not included in its configured skip list.

In particular, it extends codespell coverage to:

• Root-level markdown files (*.md)
• .github/ directory

Motivation: PR #1950 highlighted that root markdown files were not being spellchecked. This change ensures documentation in the repository root and GitHub workflows are spellchecked in CI.

This change will potentially annoy people with a lot of stray files in their repo.

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md. skipped
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.33%. Comparing base (d618e52) to head (cf061db).
⚠️ Report is 6 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #1966   +/-   ##
================================================
  Coverage            86.33%   86.33%           
================================================
  Files                  538      538           
  Lines                34557    34557           
  Branches              3222     3222           
================================================
  Hits                 29835    29835           
  Misses                4148     4148           
  Partials               574      574           

Flag	Coverage Δ
unittests	86.33% <ø> (ø)

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