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>

CHANGELOG.md

@@ -24,6 +24,14 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
   category rail and scrolling carded panel that later work fills in; closes on
   ✕, Esc, or a barrier tap. (T-445)
 
+- **Schema-driven settings engine.** Subsystems register a `SettingsCategory`
+  (a `SettingsCategoryContribution` routed into the kernel `SettingsRegistry`);
+  the panel renders it as carded sections of field rows — toggle, select,
+  text, number, and an "opens external file" button — each bound to a
+  `SettingsStore` key with help text and reset-to-default. Inputs recede to
+  `panelBackground` inside the elevated `panelHeader` cards (ui-design
+  surface.md). Categories are data; registering one surfaces a new tab. (T-448)
+
 ### Changed
 
 - **Theme picker relabelled "Theme…".** The ⌘K theme picker's command title