feat(settings): schema-driven settings engine (T-448)

The core of the settings panel (epic T-444). Subsystems describe a category
as data — a SettingsCategory of carded SettingsSections of SettingsFields
(toggle / select / text / number / opens-external-file), each bound to a
SettingsStore key with help text, a default, and reset-to-default.

Registration is declarative: a new SettingsCategoryContribution carries the
category; the extension manager routes it into a new kernel SettingsRegistry
(exposed on KernelServices), which the panel reads via ClideKernel. Adding a
category is now pure data + a contribution — no widget code.

SettingsCategoryView renders a category into carded sections per ui-design
surface.md: panelHeader card fill, dividerColor border, inputs receding to
panelBackground; select reuses the anchored-overlay menu, text/number commit
on Enter or blur (numeric clamps to bounds). The modal panel now shows the
selected/first registered category, falling back to the empty state.

Tests: registry (sort / dedup / notify), contribution routing on activation,
renderer (render + toggle/select write-through + reset), modal-with-category.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/extension/src/contribution.dart

@@ -1,5 +1,6 @@
 import 'package:clide/clide.dart';
 import 'package:clide/kernel/src/panels/slot_id.dart';
+import 'package:clide/kernel/src/settings_schema.dart';
 import 'package:flutter/widgets.dart';
 
 /// One atom contributed by a [ClideExtension]. Extensions declare N of
@@ -138,3 +139,13 @@ class LayoutSlot {
   final double? maxSize;
   final bool visible;
 }
+
+/// A category in the Settings panel (epic T-444). The kernel routes it into the
+/// `SettingsRegistry`; `builtin.settings-ui` renders its [SettingsCategory]
+/// schema into carded field rows (T-448). Categories are data — register one to
+/// surface a new settings tab.
+class SettingsCategoryContribution extends ContributionPoint {
+  const SettingsCategoryContribution({required super.id, required this.category});
+
+  final SettingsCategory category;
+}