Add a REST endpoint for saved route preferences

Only folders inside the active working directory can be opened.

Resolving folder access limits

I can't reach the folder path /project/station/unset, because tools are limited to /project/station. The repository should be mounted at the station root before the run starts, and the endpoint work can only begin once that path resolves to a checkout the agent is allowed to read. The safest next move is to confirm which checkout the session was handed, then re-run the folder listing against that path rather than guessing at a sibling directory, which would fail the same way. If the listing still comes back empty after that, the run should stop and surface the mount error rather than scaffolding an endpoint against a tree it cannot read.