feat(comments): add lexicons for comment create/update/delete XRPC procedures
Define AT Protocol lexicon schemas for comment write operations:
- social.coves.community.comment.create: Create new comments/replies
- social.coves.community.comment.update: Update existing comment content
- social.coves.community.comment.delete: Delete comments by URI
All procedures require OAuth authentication and follow atProto conventions
with proper error definitions (ContentTooLong, ContentEmpty, NotAuthorized, etc.)
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Co-Authored-By: Claude <noreply@anthropic.com>