feat: delegate storage:exec to scheduler plugins, harden the flow

Moves the actual exec out of the storage plugin and into a new scheduler-storage-exec plugn trigger. scheduler-docker-local does docker run with TTY-aware -it/-i selection and --user derived from entry.Chown; scheduler-k3s creates a throwaway Pod via the kubernetes API, waits for it to reach Running with structured error reporting (ImagePullBackOff and friends are surfaced from the container status verbatim, no kubectl involvement), execs the user command via the existing SPDY plumbing in k8s.go, and deletes the Pod on the way out. (Entry).Validate now accepts either an absolute path or a docker named-volume token for docker-local entries so the migration synthesizer's named-volume legacy entries work cleanly. storage:exec gains --as-user for one-off uid overrides, propagates the underlying tool's exit code via os.Exit, and detects TTY/interactive mode from os.Stdin so non-interactive scripted use no longer trips over docker's input-device-is-not-a-tty error.

tests/unit/storage.bats

@@ -299,6 +299,29 @@ teardown() {
   assert_success
 }
 
+@test "(storage) storage:exec runs a non-interactive command and propagates exit code" {
+  run /bin/bash -c "dokku storage:create rdmtest-exec"
+  assert_success
+
+  # docker pull image up-front so the test isn't gated on registry latency.
+  run /bin/bash -c "docker pull alpine:3 >/dev/null"
+  assert_success
+
+  run /bin/bash -c "dokku storage:exec rdmtest-exec -- /bin/sh -c 'touch /data/marker && ls /data/marker'"
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+  assert_output_contains "/data/marker"
+
+  run /bin/bash -c "dokku storage:exec rdmtest-exec -- /bin/sh -c 'exit 42'"
+  echo "output: $output"
+  echo "status: $status"
+  assert_equal "$status" 42
+
+  run /bin/bash -c "dokku storage:destroy rdmtest-exec"
+  assert_success
+}
+
 @test "(storage) storage:ensure-directory emits deprecation warning" {
   run /bin/bash -c "dokku storage:ensure-directory $TEST_APP 2>&1"
   echo "output: $output"