add find-in-files sidebar panel + Ctrl/Cmd+Shift+F

The find-in-files UI on top of the search.grep engine. A
FindInFilesController drives search.grep, accumulates streamed
search.match events (scoped to the active searchId, stale ids
ignored) grouped by file, and opens a match in the editor at its line.
The SearchPanelView contributes a sidebar tab: a debounced query box,
regex + case toggles, include/exclude glob fields, and a grouped
results list with the matched span highlighted.

findInFiles.open (Ctrl/Cmd+Shift+F) reveals and activates the search
tab.

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

lib/builtin/search/src/extension.dart

@@ -0,0 +1,31 @@
+import 'package:clide/builtin/search/src/search_panel_view.dart';
+import 'package:clide/extension/extension.dart';
+import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/widgets.dart';
+
+/// Find-in-files panel. Contributes a sidebar tab that runs workspace
+/// content searches through the daemon's `search.*` subsystem (the
+/// pure-Dart isolate-pool grep, D-79) and lists matches grouped by
+/// file, click-to-open at the line.
+class SearchExtension extends ClideExtension {
+  @override
+  String get id => 'builtin.search';
+  @override
+  String get title => 'Search';
+  @override
+  String get version => '0.1.0';
+  @override
+  List<String> get dependsOn => const [];
+
+  @override
+  List<ContributionPoint> get contributions => [
+        TabContribution(
+          id: 'search.findInFiles',
+          slot: Slots.sidebar,
+          title: 'Search',
+          icon: PhosphorIcons.magnifyingGlass,
+          priority: -90,
+          build: (_) => const SearchPanelView(),
+        ),
+      ];
+}