Polish first-party shell plugin docs, Hyprland rules, and bar-position seed

- omarchy-bar-settings.lua is gone: the Hyprland rules it carried for
    the old standalone bar-settings window no longer match anything,
    since settings is now a panel plugin summoned via IPC. The still-
    needed "Widget settings" sub-dialog rules move into
    omarchy-shell.lua next to the existing "Omarchy Settings" rules.

  - plugins README documents the `bar` plugin kind so third-party
    plugin authors aren't surprised to see it on the first-party
    omarchy.bar manifest. The note also calls out that they should ship
    `bar-widget`s, not replace the host bar.

  - omarchy-style-bar-position seeds from shell-defaults.json before
    mutating, so a user without a shell.json (or with a malformed one)
    no longer ends up with a stub config that blanks the bar layout.

default/quickshell/omarchy-shell/README.md

@@ -72,6 +72,7 @@ Supported `kinds`:
 | `overlay`    | A fullscreen overlay (e.g. background switcher)              |
 | `menu`       | A summoned menu surface                                      |
 | `service`    | A headless singleton, no UI                                  |
+| `bar`        | Reserved for the first-party bar host (`omarchy.bar`). Third-party plugins should ship `bar-widget`s; they do not replace the host bar. |
 
 `activation` is either `persistent` (loaded on startup, never unloaded) or
 `on-demand` (loaded by `shell summon <id>` and unloaded by `shell hide`).