Use HEAD for the active checkout pointer

Forest uses .forest/HEAD as the active checkout pointer, mirroring Git's separation between the active pointer and config files. .forest/config.yaml is the functional workspace registry, checkout forest.yaml remains checkout config, and switching checkouts rewrites only HEAD.

Consequences

• Known checkouts are registered in .forest/config.yaml; checkout directories are validated against it.
• V1 uses .forest/config.yaml, not .forest/workspace.yaml.
• forest.yaml is never the pointer file.