Get board

Fetch a single board's metadata. children is populated with the direct child boards only (one level deep). To reconstruct deeper subtrees, follow each child's board_id recursively or use GET /open/v1/boards and rebuild the tree on the client.

Returns: code=0 success; code=40001 invalid board_id or workspace context missing; code=40401 when the board does not exist in the caller's workspace.