List documents in workspace

List documents in the authenticated workspace.

Returns metadata only (no content). Use GET /documents/{id} for full content.

Pagination is cursor-based (opaque token). First request omits cursor; subsequent requests pass back the prior next_cursor. The cursor format changed in 0.3.0 from a raw document_id to opaque base64 — pre-0.3.0 cursors held across the migration return 400.