Add CI typo check to catch prose/code spelling errors

[neuromechanist]

Add a minimal GitHub Actions workflow that runs crate-ci/typos on every push and PR. Catches typos in markdown, code, commit messages before they land in published blog posts or slide decks.

Scope

• .github/workflows/typos.yml running on push to main and all PRs
• Use the official crate-ci/typos-action (minimal config, no custom rules to start)
• Add .typos.toml with a short allowlist for technical terms that look like typos (e.g., tool names) if needed
• Do not block merges initially; we can make it required after we see the first green run

Out of scope (later)

• Markdown link checker (mlc)
• Python/JS lint if we add code
• HED/BIDS vocabulary-aware spell check