Ship the model usage widget in the default bar

The widget hid itself well enough that nobody found it: it only ever
appeared if you knew the id and ran `omarchy bar plugin add`, since it was
in neither the shipped layout nor any migration.

Defaulting it on costs nothing on machines that don't use it. Panel.qml is
`visible: providers.length > 0`, and Main.qml only counts a provider that
is enabled and has actually recorded prompts, sessions, active days, or a
rate limit. A box that has never run Claude Code or Codex draws an empty
bar item, and the icon arrives on its own at the first scan that finds
usage — which is the behavior the widget already advertised.

The migration skips any config that already lists the widget in any
section, so a curated bar keeps its own placement rather than gaining a
second copy.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

config/omarchy/shell.json

@@ -38,6 +38,9 @@
         {
           "id": "omarchy.tray"
         },
+        {
+          "id": "omarchy.model-usage"
+        },
         {
           "id": "omarchy.bluetooth"
         },