docs: storage entries and migration guide for #6814

Updates persistent-storage.md to lead with the named storage entry workflow while keeping the legacy colon-form documentation intact, adds a Persistent storage section to the k3s scheduler doc, documents the storage-app-mounts, storage-create, storage-destroy, and storage-status triggers in plugin-triggers, and adds an entry to the 0.38.0 migration guide explaining the install-time migration of legacy mounts and the new DNS-1123 name validation. Bats coverage in tests/unit/storage.bats now exercises storage:create / list-entries / destroy, name validation rejections, multi-entry attachment, the destroy-while-mounted error, and the ensure-directory deprecation warning.

docs/appendices/0.38.0-migration-guide.md

@@ -22,6 +22,7 @@
 - The `docker-local` scheduler no longer queues an image for retirement when another running container of the same app still uses it. This fixes the case where a `ps:rebuild` against an image-based deploy (`git:from-image`) produced an identical-SHA image and the `dokku-retire` cron timer would log `Image ... has running containers, skipping rm` on every run. Stuck entries from prior versions are pruned automatically on the next `ps:retire` run.
 - All `:report` subcommands now accept the `--global` flag, which scopes the report to globally-configured properties. The flag composes with `--format json`, so a JSON report of global properties can be obtained via, for example, `dokku scheduler:report --global --format json`. Previously, combining `--global` with `--format json` was rejected with an "info flag" error, and `--global` on its own was treated as an unknown flag.
 - The `scheduler-k3s` plugin now manages env config and the dokku-generated image pull Secret as their own helm releases with stable names (`config-{app}` and `pull-secret-{app}`) rather than bundling them into the app helm chart with a per-deploy timestamp suffix (`env-{app}.{ts}` / `ims-{app}.{ts}`). This fixes two bugs: a helm rollback of the app chart no longer deletes Secrets that older ReplicaSets still reference, and the Deployment's `imagePullSecrets` list no longer accumulates references to nonexistent Secrets across deploys. The next deploy of an app switches the Deployment's `envFrom` and `imagePullSecrets` references to the stable names and prunes any leaked entries; existing live Deployments do not need to be patched manually. App rename now also uninstalls the old `tls-{app}`, `config-{app}`, and `pull-secret-{app}` releases under the previous app name; the new name's releases are recreated on the next deploy or certs sync.
+- The storage plugin now treats persistent volumes as named, scheduler-aware first-class resources via `storage:create`, `storage:mount`, `storage:set`, and `storage:destroy`. The legacy `storage:mount <app> <host>:<container>` colon form continues to work on docker-local apps but is deprecated; on k3s apps it is rejected. Existing colon-form mounts are migrated automatically the first time the new storage plugin runs (during the install trigger) - they appear as `legacy-<hash>` entries in `storage:list-entries`. The migration is idempotent and tied to a per-app flag file at `$DOKKU_LIB_ROOT/config/storage/.migrated/<app>`; deleting that file forces a re-scan on the next install. The `storage:ensure-directory` command keeps working but now emits a deprecation warning - prefer `storage:create <name> [<path>]` (the path defaults to the same `$DOKKU_LIB_ROOT/data/storage/<name>` location). Storage entry names must now be DNS-1123 labels of 45 characters or less so they can be used verbatim as Helm release and Kubernetes resource names; underscores and uppercase characters that the older `ensure-directory` validator accepted are rejected for new names. The migration synthesizer always uses lowercase hex hashes so existing data is never locked out.
 
 ### TLS handshake behavior change