refactor: migrate nate.rip to site.standard schema (#46)

* chore: migrate to site.standard schema

* chore: remove legacy field fallbacks in atproto.ts

Use only site.standard field names (textContent, publishedAt) without
fallbacks to old rip.nate schema fields (content, createdAt).

* fix: align with official site.standard spec

- Add site.standard.theme.color and site.standard.theme.basic lexicons
- Update document lexicon: add format:uri to site field, add content union,
  fix textContent description (plaintext only, no markdown)
- Update publication lexicon: key=tid, proper preferences ref
- Add basicTheme to publication with nate.rip dark theme colors
- Use content union for markdown, textContent for plaintext:
  - content: { $type: "rip.nate.content.markdown", text: "..." }
  - textContent: stripped plaintext for search/indexing
- Add scripts/lib/markdown.ts with stripMarkdown and markdownContent helpers
- Update all upload scripts and migration script
- Frontend reads markdown from content.text field

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Co-authored-by: GitHub Ace <githubnext@users.noreply.github.com>