Keep data outside .forest/

Forest checkout workspaces store pointers and metadata, not biological data. Data paths remain outside .forest/ so multiple repos, each with their own workspace metadata, can point to the same data without copying or moving it.

Consequences

• .forest/checkouts/<name>/ contains checkout metadata such as shared forest.yaml plus user-local local.yaml and sync_state.json.
• local.yaml stage path bindings point to real data locations.
• --adopt must preserve existing effective data paths instead of moving data into .forest/.