Export complete conversation archives with attachments

Export each conversation as a complete, portable archive. The ZIP should include conversation.md, conversation.json, metadata, AI-generated images and files, downloadable code or documents, and user-uploaded attachments. Markdown should use relative paths for local assets. Missing or expired assets should be listed in a manifest instead of being silently omitted.

Initial platform priority: ChatGPT, Claude, Grok, and Perplexity.