refactor(content): standardize on authors over redundant author key

Every content file carried both `author:` (string) and `authors:` (list)
with identical values — the singular was always equal to authors[0]. The
`authors:` list already drives the Hugo taxonomy (hugo.yaml: author:
authors); `author:` only fed the byline display in three templates.

- Repoint single/list/index bylines from `.Params.author` to
  `index .Params.authors 0`, preserving primary-author-only bylines.
- Strip the redundant `author:` key from all content frontmatter.
- Drop the `author` field from the article/podcast content types in
  frontmatter.json and promote `authors` to required so the CMS no
  longer re-adds the singular key.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: HeyItsGilbert