Split omarchy-plugin into siblings and consolidate bar config into omarchy-bar

omarchy-plugin mixed plugin lifecycle, clone/edit, and bar-layout mutation.
The bar layout was mutated by two separate engines (an embedded Python
mutate_bar here and jq in omarchy-config-shell-bar), each with its own
manifest walker.

- omarchy-plugin is now a ~220-line router; clone and edit move to
  omarchy-plugin-clone and omarchy-plugin-edit, matching the existing
  plugin-{source,add,update,remove,available,validate,scan} sibling pattern
- omarchy-config-shell-bar is replaced by omarchy-bar, the single bar engine
  (show/layout/list/options/use/reset/add/move/remove/set/replace/position/
  transparent/settings) with one jq pipeline; the Python mutate_bar and its
  python3 dependency are gone
- omarchy-plugin-catalog is the shared manifest scanner used by omarchy-bar
  (widget/option enumeration, add validation) and omarchy-plugin-clone
  (source enumeration), replacing three find|jq re-implementations
- service install/remove and refresh-shell call omarchy-bar; docs and tests
  updated to the new command surface

shell/README.md

@@ -134,7 +134,7 @@ You can still drop a plugin in without a source:
 1. Put it in `~/.config/omarchy/plugins/<plugin-id>/` with a `manifest.json`
    plus the QML referenced from its `entryPoints`.
 2. `omarchy plugin rescan`.
-3. `omarchy plugin enable <id>` (bar widgets also need `omarchy plugin bar add <id>`; full bar replacements are selected with `omarchy plugin bar use <id>`).
+3. `omarchy plugin enable <id>` (bar widgets also need `omarchy bar add <id>`; full bar replacements are selected with `omarchy bar use <id>`).
 
 The lower-level IPC equivalents remain available via `omarchy-shell shell rescanPlugins`,
 `omarchy-shell shell setPluginEnabled <id> true`, and `omarchy-shell shell listPlugins`.