Function import_brain 

pub fn import_brain(
    target_workspace: &Path,
    source_workspace: &Path,
    opts: &ImportBrainOptions,
) -> Result<ImportBrainReport>

Import knowledge from source_workspace into target_workspace.

Mode	Behavior
--as id (copy)	Copy Markdown under docs/subbrains/id/; SubBrain stays separate
--as id --mount	If source is under target, attach path as root (umbrella, no copy)
--into main	Merge copy into MainBrain tree (docs/imported/ by default)

Nested source .brain/ is not deleted; mount keeps projects shareable and independently openable. Call sync + scopes reconcile after.