test: cover the legacy -v migration with go and bats tests

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.

plugins/storage/src/commands/commands.go

@@ -25,6 +25,7 @@ Additional commands:`
     storage:info <name> [--format text|json], Show details for one storage entry
     storage:list <app> [--format text|json], List bind mounts for app's container(s) (host:container)
     storage:list-entries [--scheduler s] [--format text|json], List registered storage entries
+    storage:migrate [<app>|--all], Re-run the legacy -v to attachment migration for an app
     storage:mount <app> <host-dir:container-dir>, Create a new bind mount
     storage:report [<app>] [<flag>], Displays a storage report for one or more apps
     storage:set <name> [flags], Update a storage entry in place