Install Elsewhen, the world clock plugin, by default

Elsewhen (omacom.elsewhen) arrives as the elsewhen package under
/usr/share/omarchy/plugins, the packaged root the shell scans between its
bundled plugins and the user's. It opens the right section of the default
bar, just before the tray, and a migration installs the package, writes the
widget into a customized shell.json in the same spot, and retires a pristine
pre-package clone of the upstream repo that the package now shadows.

shell/README.md

@@ -140,6 +140,19 @@ You can still drop a plugin in without git:
    `barWidget.defaultSection`, or in the center when it is omitted, and can be
    moved with `omarchy bar move`; a full bar replaces the one in use.
 
+### Packaged plugins
+
+Some default plugins ship as their own Arch package rather than in this
+checkout: Elsewhen (`omacom.elsewhen`) is the `elsewhen` package. pacman installs such a plugin,
+root-owned, at `/usr/share/omarchy/plugins/<plugin-id>/`, and
+the shell scans that root between its bundled plugins and the user's. A
+packaged `omarchy.*` id is trusted exactly like a bundled one; a packaged
+plugin under any other id is treated like a user plugin. They update with
+`omarchy update` and are removed with pacman, so `omarchy plugin update` and
+`omarchy plugin remove` refuse them and name the package instead. A bundled
+copy in `$OMARCHY_PATH/shell/plugins` wins over the packaged one, which is how
+a packaged plugin is developed under `omarchy dev link`.
+
 The lower-level IPC equivalents remain available via `omarchy-shell shell rescanPlugins`,
 `omarchy-shell shell enablePlugin <id> '{}'`, and `omarchy-shell shell listPlugins`.
 The `omarchy plugin` commands wrap those calls. `omarchy bar move` and
@@ -224,6 +237,7 @@ customization from the shipped defaults lives in it.
 |-----------------------------------|----------------|--------------------------------------------------------|
 | `~/.config/omarchy/shell.json`    | the shell      | full layout + per-entry settings + enabled plugin list |
 | `~/.config/omarchy/plugins/<id>/` | user           | drop-in third-party plugin source files                |
+| `/usr/share/omarchy/plugins/<id>/` | pacman        | packaged plugins (e.g. `elsewhen`), updated by `omarchy update` |
 
 The `config/omarchy/shell.json` default config describes the
 fresh-install state. When the user has no `shell.json`, the shell uses