Simplify Elsewhen installation and bar migration

Link the package into the existing plugin directory and let bar put handle enablement, clock-relative placement, and the missing-clock fallback. Preserve user checkouts and existing placements, and seed the same link for new users. This removes the Atreyu packaged-discovery prerequisite and the checkout cleanup and JSON rewrite machinery.

docs/file-layout.md

@@ -78,9 +78,6 @@ 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/*