feat: align content model with site.standard for interop (#54)
* chore: add .worktrees/ to .gitignore
* feat: align content model with site.standard for interop
Three schema changes for standard.site compatibility:
1. Content union: topic.post and topic.reply `content` fields
change from bare string to open union with `$type` discriminator.
Initial type: `forum.barazo.richtext#markdown`. Enables future
content format extensibility.
2. Site field: optional `site` string on topic.post for linking
to a site.standard.publication record (at:// URI or https://).
3. Rename createdAt → publishedAt on topic.post to match
site.standard.document semantics.
New lexicon: forum.barazo.richtext (defines #markdown content type).
New Zod schema: markdownContentSchema with matching validation.
Closes singi-labs/barazo-workspace#80
BREAKING CHANGE: topic.post.content and topic.reply.content are now
union objects with $type discriminator instead of bare strings.
topic.post.createdAt renamed to publishedAt.
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