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/*
 

docs/omarchy-shell.md

@@ -88,9 +88,9 @@ bar from the CLI — `use | reset | defaults | position | transparent | put |
 move | set`, with placement flags such as `--section` and `--index`.
 The lower-level IPC methods remain available through `omarchy-shell shell ...`.
 
-## Packaged plugins
+## Elsewhen
 
-A default plugin can ship as its own Arch package instead of in the Omarchy checkout: Elsewhen (`omacom.elsewhen`) is the `elsewhen` package. pacman installs it at `/usr/share/omarchy/plugins/<id>/`, a root the shell scans between `$OMARCHY_PATH/shell/plugins` and `~/.config/omarchy/plugins`. A packaged `omarchy.*` id is trusted like a bundled plugin and loads by default; any other packaged id behaves like an installed plugin. Precedence is bundled, then packaged, then user, so a checkout under `omarchy dev link` overrides the package. Packaged plugins update through `omarchy update` and leave through pacman; `omarchy plugin update` and `omarchy plugin remove` refuse them and name the package, and Setup › Plugins › Remove does not list them.
+Elsewhen (`omacom.elsewhen`) ships in the `elsewhen` package at `/usr/share/omarchy/plugins/omacom.elsewhen`. A symlink in `~/.config/omarchy/plugins/` makes it available to the shell. New installs place it immediately before the clock; the migration uses `omarchy bar put omacom.elsewhen --before omarchy.clock`, which preserves an existing placement and uses Elsewhen's normal right-side placement if the clock is absent. Existing plugin directories and symlinks are left intact.
 
 ## IPC