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.

default/agents/skills/omarchy/plugins.md

@@ -9,9 +9,14 @@ inside a single long-running Quickshell process (`omarchy-shell`).
 ```
 ~/.config/omarchy/shell.json             # User overrides: bar, plugins, idle
 ~/.config/omarchy/plugins/<plugin-id>/   # User-owned shell plugins
+/usr/share/omarchy/plugins/<plugin-id>/  # Packaged plugins (pacman), e.g. omacom.elsewhen
 $OMARCHY_PATH/config/omarchy/shell.json  # Canonical defaults
 ```
 
+Packaged plugins such as Elsewhen (`elsewhen` package) update with `omarchy update` and are removed with pacman;
+`omarchy plugin update` and `omarchy plugin remove` refuse them and name the
+package.
+
 The shell hot-reloads `shell.json` on save — no restart needed for layout
 changes. `idle.screensaver` and `idle.lock` are seconds since user idle began.