Port the AGS widgets into the shell as plugins

docs/widgets.md

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+# Ported widgets
+
+The GTK widgets that used to run as a separate AGS process are now plugins
+inside the shell. A panel opens over IPC into a process that is already running
+rather than starting a second toolkit, and `ags` is no longer installed or
+autostarted.
+
+## Where each one went
+
+| Was | Is | Note |
+| --- | --- | --- |
+| `QuickSettings.tsx` (523 lines) | `shell/plugins/panels/quick-settings/` | split across 9 files, none over 250 lines |
+| `SysMonitor.tsx` (142) | `shell/plugins/panels/sysmon/` | CPU, memory, disk, temperature |
+| `Notifications.tsx` (160) | `shell/plugins/notification-center/` | reads the notification service's own history files |
+| `WallPicker.tsx` (106) | the existing image picker | `blob-wallpaper-set --menu` now calls `blob-menu-images`, so wallpapers get the image grid instead of a text list |
+| `ThemePicker.tsx` (178) | the menu and `blob-theme-switcher` | the shell already had a theme switcher; a second picker was duplicate surface |
+| `ClaudeUsage.tsx` (372) | dropped | it existed to read agent usage records, and the agents feature is gone |
+| `Media.tsx` (104) | dropped | `SHOW_DESKTOP_MEDIA` was already false, and the quick settings media row covers it |
+| `WidgetCard.tsx` (22) | `Ui/Card.qml`, `Ui/CardHeader.qml`, `Ui/StatRow.qml` | shared, so no plugin repeats the card |
+| `style.css` (759) | `Commons/Style.qml` tokens | see below |
+
+## The look
+
+`ags/style.css` was the visual identity worth keeping: square corners, a 2px
+border, and an `alpha(background, 0.6)` fill. Those are now named tokens on
+`Style` - `cardBorderWidth`, `cardRadius`, `cardFillAlpha`, `cardBorderAlpha`,
+`panelFillAlpha` - so every ported panel reads the same numbers instead of
+repeating literals.
+
+The CSS was written against pywal's `@color0`, `@color4` and `@color5` aliases.
+`blob-theme-color` resolves `color4` to blue and `color5` to magenta, so
+`Commons/Color.qml` now exposes `blue` and `magenta` with that same cascade,
+falling back to `accent` for themes that define neither. `color0` is
+`background`, which Color already had.
+
+## Opening them
+
+| Panel | Opened by |
+| --- | --- |
+| Quick settings | left-click the bar clock, or `Super + Ctrl + Q` |
+| Notification centre | left-click the bar bell |
+| System monitor | `Super + Ctrl + M` |
+
+Both new bar click actions are already in `shell.json`. The system monitor had no
+bar entry point in the AGS setup either, so the keybinding is how it is reached.
+
+## Deliberately not ported
+
+The tiles still poll shell commands the way the GTK widgets did - `pamixer` for
+volume, `brightnessctl` for brightness, `playerctl` for media - rather than
+binding to the shell's own audio, brightness and media services. Polling only
+runs while a panel is open, so the cost is bounded, but wiring these to the
+services is the obvious next refinement and would remove the last per-tick
+process spawns.