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.

shell/README.md

@@ -140,10 +140,6 @@ 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
@@ -228,7 +224,6 @@ 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