Contributor avatars: standardize on `avatar` frontmatter (docs and broken profiles)

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Problem
Contributor avatars fail to render when profile frontmatter uses avatar_url. Our docs also recommend avatar_url, causing new profiles to break. Additionally, the anomaly namespace references a .png that doesn’t exist; the file is .jpeg.

Impact

• Broken contributor avatars on the site
• Confusing/incorrect docs leading to repeated mistakes

Scope observed

• Namespaces: anomaly, sharkymark, ericpaulsen
• Docs: MAINTAINER.md, CONTRIBUTING.md

Proposed fix

• Standardize frontmatter on avatar (not avatar_url)
• Update docs to reflect the correct key
• Fix anomaly profile to reference ./.images/avatar.jpeg

Existing work

• There is an open PR that attempts to fix this in entirety: Fix contributor avatars and docs: use avatar key and correct anomaly image extension #312

Acceptance criteria

• All affected contributor pages render avatars correctly
• Docs (MAINTAINER.md, CONTRIBUTING.md) show avatar in examples and guidance
• anomaly namespace references the correct image extension

Verification steps

1. Build and run the registry site
2. Verify contributor avatars render on list and profile pages for Jay (anomaly), Mark (sharkymark), and Eric (ericpaulsen)
3. Confirm docs show avatar frontmatter and no references to avatar_url remain