Export data from SaaS to local/docker (md export + import/embedding regen)

Summary:
A user in Discord asked about exporting app.getcore (SaaS) data to a local Docker install. Today there’s no one-click export. Proposed approach is to export all documents as Markdown and provide a local import path; optionally provide a script/API to transfer/recreate embeddings. Note that embedding model differences in local installs may change similarity search results, so MD-based export/import plus embedding regeneration is preferred. Follow-up in thread asks if there is an API/script to fetch embeddings and effectively enable a 'transfer of brain'.

Discord thread: https://discord.com/channels/1347867796624900206/1470655117165461566

Acceptance criteria:

1. Markdown export from SaaS for all docs/spaces, including metadata (ids, created/updated timestamps, hierarchy).
2. Local import from Markdown into Docker/local install.
3. Embedding regeneration during import.
4. Optional script/API for bulk export/import.
5. Documentation/instructions for end users.