ci: add typos-cli spelling check

[ColumbusLabs]

Closes #368.

This adds a dedicated typos-cli check for pull requests and pushes to main. The workflow pins both actions to immutable commits and grants only read access to repository contents.

The initial local run found two genuine documentation misspellings, which this PR also fixes. _typos.toml recognizes the project's BA abbreviation so the checker remains focused on actionable findings.

Verification:

• typos (v1.49.0) passes across the full repository.
• actionlint .github/workflows/typos.yml passes.
• mdbook build completes with the repository's pinned mdBook plugins.
• git diff --check passes.

[vados-cosmonic]

LGTM 🚀

Thanks for the contributoin @ColumbusLabs -- this is just what we needed.