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>

test/kernel/src/keymap/intents_test.dart

@@ -35,6 +35,10 @@ void main() {
       expect(parseIntentId('quickOpen.accept'), isA<QuickOpenAcceptIntent>());
     });
 
+    test('returns the findInFiles.open intent', () {
+      expect(parseIntentId('findInFiles.open'), isA<FindInFilesIntent>());
+    });
+
     test('returns the text.scale* intents', () {
       expect(parseIntentId('text.scaleIncrease'), isA<TextScaleIncreaseIntent>());
       expect(parseIntentId('text.scaleDecrease'), isA<TextScaleDecreaseIntent>());