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>

shell/plugins/model-usage/README.md

@@ -29,6 +29,11 @@ there is no switch row at all; with none, the module leaves the bar entirely
 rather than sitting there with nothing to say. A CLI installed mid-session
 shows up at the next refresh, so nothing polls the disk waiting for it.
 
+That self-hiding is why the widget ships in the default bar layout: a machine
+that has never run Claude Code or Codex draws nothing, and the icon arrives on
+its own the first time a scan finds usage. Drop it with
+`omarchy bar plugin remove omarchy.model-usage`.
+
 ## Providers
 
 | Provider | Limits | Local stats |