- Add CloudFrontSigner.SignedURL() for generating per-resource signed URLs
- Attachment responses include download_url (5-min signed URL for CLI)
- Eager load attachments on comments and timeline (same pattern as reactions)
- Add ListAttachmentsByCommentIDs query for batch loading
- Update Comment and TimelineEntry types with attachments field
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>