Function apply 

pub fn apply(
    store: &LocalFsStore,
    flags: &OpFlags,
    target: &str,
) -> Result<OpOutcome, OpError>

op env migrate-dev <target> --apply. Re-runs the check, refuses with OpError::Conflict if not clean, then merges the source env into the target under the target’s transact lock and renames the source directory to a hidden .dev-migrated-<ts> sentinel.

Idempotent: if no source env exists, returns MigrateDevApplyOutcome with empty merged_slots and legacy_dir_renamed_to: None.