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/kernel/src/settings_schema.dart

@@ -0,0 +1,103 @@
+/// Schema model for the settings panel (T-448, epic T-444).
+///
+/// Pure data — no Flutter imports — so any subsystem can declare a category
+/// without depending on the widget layer. The settings-ui renderer turns a
+/// [SettingsCategory] into carded sections of field rows; each field binds to
+/// a `SettingsStore` key and is read/written through the store.
+library;
+
+/// The control a [SettingsField] renders as.
+enum SettingsFieldKind {
+  /// On/off boolean.
+  toggle,
+
+  /// One value chosen from [SettingsField.options].
+  select,
+
+  /// Free-text input.
+  text,
+
+  /// Numeric input (optionally bounded by [SettingsField.min]/[max]).
+  number,
+
+  /// A row that opens an external file/editor (e.g. `.editorconfig`) instead
+  /// of editing a value inline — the action is a command id, keeping the
+  /// schema widget-free.
+  file,
+}
+
+/// One choice in a [SettingsFieldKind.select] field.
+class SettingsOption {
+  const SettingsOption({required this.value, required this.label});
+
+  /// Stored value.
+  final String value;
+
+  /// Human label shown in the picker.
+  final String label;
+}
+
+/// One editable setting. [key] is a `SettingsStore` key — its `app.`/
+/// `project.`/`ext.` prefix determines the scope (and the per-field scope tag,
+/// T-449). The renderer reads the current value with `store.get`, falling back
+/// to [defaultValue] when unset, and writes edits with `store.set`.
+class SettingsField {
+  const SettingsField({
+    required this.key,
+    required this.kind,
+    required this.label,
+    this.help,
+    this.defaultValue,
+    this.options = const [],
+    this.min,
+    this.max,
+    this.fileCommand,
+  });
+
+  final String key;
+  final SettingsFieldKind kind;
+  final String label;
+
+  /// Optional one-line help shown under the label.
+  final String? help;
+
+  /// Value shown / restored when the key is unset (reset-to-default target).
+  final Object? defaultValue;
+
+  /// Choices for [SettingsFieldKind.select].
+  final List<SettingsOption> options;
+
+  /// Optional inclusive bounds for [SettingsFieldKind.number].
+  final num? min;
+  final num? max;
+
+  /// For [SettingsFieldKind.file]: the command id the row's button invokes.
+  final String? fileCommand;
+}
+
+/// A carded group of fields (surface.md "sectioned cards"). [label] is the
+/// small-caps header rendered just above the card.
+class SettingsSection {
+  const SettingsSection({required this.label, required this.fields});
+
+  final String label;
+  final List<SettingsField> fields;
+}
+
+/// One settings category — a rail entry (T-447) plus the sections its panel
+/// shows. Subsystems register these via `SettingsCategoryContribution`; the
+/// renderer draws them.
+class SettingsCategory {
+  const SettingsCategory({required this.id, required this.title, required this.sections, this.iconName, this.priority = 0});
+
+  final String id;
+  final String title;
+
+  /// Phosphor glyph name, resolved via `PhosphorIcons.byName` at render (T-314).
+  final String? iconName;
+
+  /// Rail ordering — lower sorts first; ties broken by [title].
+  final int priority;
+
+  final List<SettingsSection> sections;
+}