Simplify omarchy-bar into a bar-settings and bar-plugin split

The omarchy-bar command had grown three overlapping ways to inspect the
bar (show/layout/list/options, plus selected/active/available/widgets
aliases) and mixed layout mutation in with bar-level settings. Untangle
it into two focused commands:

- omarchy-bar keeps only the bar-level settings that write shell.json:
  use, reset, position, transparent, and settings. reset now delegates
  to `use omarchy.bar` rather than duplicating the del(.bar.id) write.
- omarchy-bar-plugin owns all layout mutation: add, move, remove, set,
  and replace, with the placement flags and jq resolve/anchor helpers.
  `omarchy bar plugin ...` routes here via the dispatcher.

Drop the inspection commands entirely: the layout is visible on the bar,
the config is shell.json, and widget/option ids come from
`omarchy plugin list`. Nothing consumed the show output programmatically
except tests. This also removes omarchy-bar-position, whose jq write was
a duplicate of `omarchy bar position`.

Strip environment-invariant guards (require_command, require_omarchy_path)
that defended against jq or OMARCHY_PATH being absent — neither happens on
a real system. Keep the user-input validation (--section/--index) and the
atomic shell.json write.

Update callers (service install/remove, refresh-shell, plugin-clone,
plugin enable), keybindings, the menu, tests, and docs to the new split.

shell/README.md

@@ -131,7 +131,7 @@ You can still drop a plugin in without git:
 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 bar add <id>`; full bar replacements are selected with `omarchy bar use <id>`).
+3. `omarchy plugin enable <id>` (bar widgets also need `omarchy bar plugin 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`.