migrate group cards onto ClideCollapserCard; retire ClideHolderCard (T-305)

The activity run, edit run, and sub-agent run cards now render through the
shared ClideCollapserCard primitive instead of ClideHolderCard. The
collapsed ticker now leads with the card label, the count sits in a
fixed-width slot, and the status tick is pinned to the right edge.

ClideHolderCard (T-266) is fully superseded — removed along with its test
and golden; the deeper-control-passthrough coverage moved to the
ClideCollapserCard widget test.

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

test/goldens/holder_card_goldens_test.dart

@@ -1,66 +0,0 @@
-import 'package:alchemist/alchemist.dart';
-import 'package:clide/builtin/claude/src/conversation_card.dart';
-import 'package:clide/builtin/claude/src/holder_card.dart';
-import 'package:flutter/widgets.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-import '../helpers/kernel_fixture.dart';
-import '../helpers/widget_harness.dart';
-
-void main() {
-  late KernelFixture f;
-
-  setUp(() async => f = await KernelFixture.create());
-  tearDown(() async => f.dispose());
-
-  goldenTest(
-    'ClideHolderCard (T-266): collapsed ticker + expanded container of sub-cards',
-    fileName: 'holder_card',
-    builder: () => GoldenTestGroup(
-      columns: 1,
-      children: [
-        GoldenTestScenario(
-          name: 'collapsed (ticker + step count)',
-          child: _wrap(
-            f,
-            const ClideHolderCard(
-              collapsedSummary: 'Bash  ls -la',
-              stepLabel: '3 steps',
-              children: [],
-            ),
-          ),
-        ),
-        GoldenTestScenario(
-          name: 'expanded (framed container wrapping sub-cards)',
-          child: _wrap(
-            f,
-            const ClideHolderCard(
-              collapsedSummary: 'Bash  ls -la',
-              stepLabel: '3 steps',
-              initiallyExpanded: true,
-              children: [
-                ConversationCard(
-                  variant: ConversationCardVariant.bordered,
-                  accent: Color(0xFF4C9AFF),
-                  label: 'Bash',
-                  body: Text('ls -la', textDirection: TextDirection.ltr),
-                ),
-                ConversationCard(
-                  variant: ConversationCardVariant.bordered,
-                  accent: Color(0xFF4C9AFF),
-                  label: 'Read',
-                  body: Text('/lib/main.dart', textDirection: TextDirection.ltr),
-                ),
-              ],
-            ),
-          ),
-        ),
-      ],
-    ),
-  );
-}
-
-Widget _wrap(KernelFixture f, Widget child) => SizedBox(
-      width: 360,
-      child: harness(f, child),
-    );