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/main.dart

@@ -12,6 +12,7 @@ import 'package:clide/builtin/editor/editor.dart';
 import 'package:clide/builtin/extensions_ui/extensions_ui.dart';
 import 'package:clide/builtin/files/files.dart';
 import 'package:clide/builtin/git/git.dart';
+import 'package:clide/builtin/search/search.dart';
 import 'package:clide/builtin/grammars_core/grammars_core.dart';
 import 'package:clide/builtin/graph/graph.dart';
 import 'package:clide/builtin/ipc_status/ipc_status.dart';
@@ -246,6 +247,7 @@ Future<void> main() async {
     ..register(TicketsExtension())
     ..register(DecisionsExtension())
     ..register(FilesExtension())
+    ..register(SearchExtension())
     ..register(GitExtension())
     ..register(PqlExtension())
     ..register(ProblemsExtension())