Adds plugins/storage/migrate_test.go exercising migrateApp and MigrateLegacyMounts: single deploy phase, cross-phase grouping into one attachment, :ro and free-form volume options preserved on the attachment, idempotency on a second pass, the per-app flag-file fast path that skips already-migrated apps, and the name-collision refusal that aborts before draining. tests/unit/storage.bats gains an end-to-end case that stages a -v line via docker-options:add, runs storage:migrate, and asserts the synthesized colon form shows up in storage:list, the legacy-<hash> entry shows up in storage:list-entries, and the original -v line is gone from docker-options:report on both phases. To make that test (and operators with restored backups) practical, storage gains a storage:migrate <app>|--all subcommand backed by a new MigrateApp helper that skips the per-app flag file so a re-run actually re-scans.
apps:destroy was failing in CI because the post-delete trigger called PropertyListWrite, which tried to mkdir /var/lib/dokku/config/storage/<app> as the dokku user but the parent directory had been created by the install trigger as root. The install trigger now calls PropertySetup("storage") so the per-plugin config root has the correct ownership, and post-delete / post-app-clone / post-app-rename short-circuit when the app has no attachments to begin with so they can't fail on a never-touched property tree. The entry registry and migration flag directories also moved out of config/storage (where they would have collided with the property-store path config/storage/<appName>) into a dedicated data/storage-registry tree.
storage:mount and storage:unmount now accept the named-entry form (--container-dir, --phase, --process-type, --volume-subpath, --volume-readonly, --volume-chown), persisting attachments via the property list. The legacy host:container colon form is preserved for back-compat. Storage now emits its own docker-args-deploy and docker-args-run triggers so docker-local apps continue to receive bind mounts even after their data moves out of docker-options. The install trigger walks every app once per upgrade and converts legacy -v lines into legacy-<hash> entries plus attachments, draining the originals from docker-options last so a partial run leaves the old behavior intact. Lifecycle triggers post-delete, post-app-clone-setup, and post-app-rename-setup keep the attachment store in sync with app state.
