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.

docs/file-layout.md

@@ -78,6 +78,9 @@ install/**                     ──►  omarchy             /usr/share/omarchy
 migrations/**                  ──►  omarchy             /usr/share/omarchy/migrations/
 themes/**                      ──►  omarchy             /usr/share/omarchy/themes/
 shell/**                       ──►  omarchy             /usr/share/omarchy/shell/
+                                                        (packaged plugins from their own
+                                                        packages, e.g. elsewhen, land beside it
+                                                        in /usr/share/omarchy/plugins/<id>/)
 version                        ──►  omarchy             /usr/share/omarchy/version
                                                         + /etc/skel/.local/state/omarchy/migrations/*