feat(settings): Extensions tab — "watch this space" notice (T-456)

Built-in extensions are always on and there's no third-party install path yet,
so the Extensions tab is a notice rather than a toggle list — a toggle list
would let users brick the app by disabling load-bearing builtins. It explains
that installing/enabling/disabling arrives with third-party (Lua) extensions
and points at the records that pick it up (D-16 / T-8).

Fleshes out the builtin.extensions-ui stub via the custom-control hatch. Adds
two renderer guards — empty section/field labels render no chrome — so a
notice-only category shows just its card.

Tests: contributes the Extensions category + notice control; the notice renders.

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

lib/builtin/extensions_ui/extensions_ui.dart

@@ -1 +1,2 @@
 export 'src/extension.dart';
+export 'src/extensions_notice.dart';

lib/builtin/extensions_ui/src/extension.dart

@@ -1,17 +1,39 @@
+import 'package:clide/builtin/extensions_ui/src/extensions_notice.dart';
 import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
 
-/// Tier-0 stub. Real implementation lands in a later tier; the extension
-/// is registered so the extensions-ui surface can list it as "installed,
-/// not yet implemented" and its id is reserved.
+/// Extensions settings tab (T-456). Built-in extensions are always on and there
+/// is no third-party install path yet, so the tab is a "watch this space"
+/// notice pointing at the records that track extension management (D-16 / T-8)
+/// rather than a toggle list. The real enable/install UI lands with third-party
+/// (Lua) extensions.
 class ExtensionsUiExtension extends ClideExtension {
   @override
   String get id => 'builtin.extensions-ui';
   @override
   String get title => 'Extensions UI';
   @override
-  String get version => '0.0.0-stub';
+  String get version => '0.1.0';
   @override
   List<String> get dependsOn => const [];
+
   @override
-  List<ContributionPoint> get contributions => const [];
+  List<ContributionPoint> get contributions => [
+    SettingsControlContribution(id: 'extensions-ui.notice-control', customId: 'extensions.notice', builder: (_) => const ExtensionsNotice()),
+    const SettingsCategoryContribution(
+      id: 'extensions',
+      category: SettingsCategory(
+        id: 'extensions',
+        title: 'Extensions',
+        iconName: 'puzzle-piece',
+        priority: 80,
+        sections: [
+          SettingsSection(
+            label: '',
+            fields: [SettingsField(key: 'app.extensions._notice', kind: SettingsFieldKind.custom, label: '', customId: 'extensions.notice')],
+          ),
+        ],
+      ),
+    ),
+  ];
 }

lib/builtin/extensions_ui/src/extensions_notice.dart

@@ -0,0 +1,56 @@
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/widgets.dart';
+
+/// The Extensions settings tab's "watch this space" notice (T-456).
+///
+/// Built-in extensions are always on and there's no third-party install path
+/// yet, so there's nothing to manage. Rather than ship a toggle list that could
+/// brick the app, the tab explains that extension management arrives with
+/// third-party (Lua) extensions and points at the records that track it.
+/// Rendered inside the section card (no own surface) via the custom-control
+/// registry.
+class ExtensionsNotice extends StatelessWidget {
+  const ExtensionsNotice({super.key});
+
+  static const ns = 'builtin.extensions-ui';
+
+  @override
+  Widget build(BuildContext context) {
+    final tokens = ClideTheme.of(context).surface;
+    final i = ClideKernel.of(context).i18n;
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Row(
+          children: [
+            ClideIcon(PhosphorIcons.byName('puzzle-piece'), size: 18, color: tokens.globalTextMuted),
+            const SizedBox(width: 8),
+            ClideText(
+              i.string('notice.title', namespace: ns, placeholder: 'Extension management is coming'),
+              fontSize: 14,
+              fontWeight: FontWeight.w600,
+            ),
+          ],
+        ),
+        const SizedBox(height: 10),
+        ClideText(
+          i.string(
+            'notice.body',
+            namespace: ns,
+            placeholder:
+                'Installing, enabling, and disabling extensions arrives with third-party (Lua) extension support. For now the built-in extensions are always on.',
+          ),
+          color: tokens.globalTextMuted,
+        ),
+        const SizedBox(height: 10),
+        ClideText(
+          i.string('notice.tracked', namespace: ns, placeholder: 'Tracked in T-8 (Tier 6) · D-16'),
+          color: tokens.globalTextMuted,
+          fontSize: clideFontCaption,
+          fontFamily: clideMonoFamily,
+        ),
+      ],
+    );
+  }
+}

lib/builtin/settings_ui/src/settings_category_view.dart

@@ -125,10 +125,11 @@ class _SectionCard extends StatelessWidget {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: [
-          Padding(
-            padding: const EdgeInsets.only(left: 2, bottom: 6),
-            child: ClideText(section.label.toUpperCase(), fontSize: clideFontCaption, color: tokens.sidebarSectionHeader, fontFamily: clideMonoFamily),
-          ),
+          if (section.label.isNotEmpty)
+            Padding(
+              padding: const EdgeInsets.only(left: 2, bottom: 6),
+              child: ClideText(section.label.toUpperCase(), fontSize: clideFontCaption, color: tokens.sidebarSectionHeader, fontFamily: clideMonoFamily),
+            ),
           ClideSurface(
             // Card surface (surface.md): panelHeader resolves to the `surface`
             // palette key (the elevated card tone); inputs inside recede to
@@ -181,8 +182,7 @@ class _FieldRow extends StatelessWidget {
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.stretch,
           children: [
-            label,
-            const SizedBox(height: 10),
+            if (field.label.isNotEmpty) ...[label, const SizedBox(height: 10)],
             builder?.call(context) ?? const SizedBox.shrink(),
           ],
         ),

lib/kernel/src/i18n/catalog/builtin.extensions-ui_en_us.json

@@ -0,0 +1,5 @@
+{
+  "notice.title": { "translation": "Extension management is coming" },
+  "notice.body": { "translation": "Installing, enabling, and disabling extensions arrives with third-party (Lua) extension support. For now the built-in extensions are always on." },
+  "notice.tracked": { "translation": "Tracked in T-8 (Tier 6) · D-16" }
+}