fold sub-agent prompt into the Agent card; relabel you → agent prompt (T-263)

A sidechain sub-agent prompt was rendered with the blue "you" label,
falsely implying the user typed it. Now:

- transcript_reader parses parentUuid (was dropped) onto every
  ConversationItem.
- conversation_view resolves each sidechain prompt to its spawning
  Agent/Task card via parentUuid (nearest-preceding Agent as fallback),
  folds the prompt into that card as a collapsed "prompt" segment, and
  suppresses the standalone block. Layered order when expanded:
  call → prompt → result (note E).
- A sidechain UserMessage never gets the "you" treatment: folded into its
  card, or — when orphaned — rendered as a muted standalone "agent prompt".

Tests: parentUuid parse, fold + suppression, parallel-agent attachment
(would fail under a nearest-preceding heuristic), orphan relabel, and a
golden for the Agent card's call → prompt → result layering.

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

test/builtin/claude/conversation_view_test.dart

@@ -282,6 +282,54 @@ void main() {
       expect(find.text('thanks'), findsOneWidget);
     });
 
+    testWidgets('a sidechain prompt folds into its Agent card; never labelled "you" (T-263)', (tester) async {
+      await pumpWith(tester, [
+        AssistantToolUse(
+            uuid: 'agt-msg', timestamp: _t, isSidechain: false, toolUseId: 'task1', name: 'Task', input: const {'description': 'explore the codebase'}),
+        UserMessage(uuid: 'p1', timestamp: _t, isSidechain: true, text: 'find all the widgets'),
+      ]);
+      // Never the blue "you", and no standalone block (folded → suppressed).
+      expect(find.text('you'), findsNothing);
+      expect(find.text('agent prompt'), findsNothing); // not a standalone card here
+      expect(find.text('Task'), findsOneWidget);
+      // Collapsed by default: the prompt is hidden.
+      expect(find.text('find all the widgets'), findsNothing);
+
+      // Expand the Agent card → a "prompt" segment reveals the folded prompt.
+      await tester.tap(find.bySemanticsLabel('Expand'));
+      await tester.pumpAndSettle();
+      expect(find.text('prompt'), findsOneWidget); // segment sub-label
+      expect(find.text('find all the widgets'), findsOneWidget);
+    });
+
+    testWidgets('an orphan sidechain prompt renders as muted "agent prompt", never "you" (T-263)', (tester) async {
+      // No Agent tool-use to attach to → stays standalone, but relabelled.
+      await pumpWith(tester, [
+        UserMessage(uuid: 'orphan', timestamp: _t, isSidechain: true, text: 'orphaned agent instructions'),
+      ]);
+      expect(find.text('you'), findsNothing);
+      expect(find.text('agent prompt'), findsOneWidget);
+    });
+
+    testWidgets('parallel agents: each prompt attaches to its own card via parentUuid (T-263)', (tester) async {
+      // Document order scrambles the prompts so a nearest-preceding heuristic
+      // would misattach BOTH to agent B; parentUuid must route them correctly.
+      await pumpWith(tester, [
+        AssistantToolUse(uuid: 'mA', timestamp: _t, isSidechain: false, toolUseId: 'tA', name: 'Task', input: const {'description': 'agent A'}),
+        AssistantToolUse(uuid: 'mB', timestamp: _t, isSidechain: false, toolUseId: 'tB', name: 'Task', input: const {'description': 'agent B'}),
+        UserMessage(uuid: 'pB', timestamp: _t, isSidechain: true, parentUuid: 'mB', text: 'PROMPT FOR B'),
+        UserMessage(uuid: 'pA', timestamp: _t, isSidechain: true, parentUuid: 'mA', text: 'PROMPT FOR A'),
+      ]);
+      // Two collapsed Agent cards; the first Expand caret belongs to card A.
+      expect(find.text('you'), findsNothing);
+      await tester.tap(find.bySemanticsLabel('Expand').first);
+      await tester.pumpAndSettle();
+      // Only card A is expanded → its prompt (A) shows; B's stays folded away.
+      // Nearest-preceding would have put A's prompt under B, revealing nothing.
+      expect(find.text('PROMPT FOR A'), findsOneWidget);
+      expect(find.text('PROMPT FOR B'), findsNothing);
+    });
+
     testWidgets('a permission-prompted tool-use is hidden but its result is kept', (tester) async {
       await pumpWith(
         tester,

test/builtin/claude/transcript_reader_test.dart

@@ -361,6 +361,23 @@ void main() {
       final items = parseAll([raw]);
       expect(items.first.isSidechain, isTrue);
     });
+
+    test('parentUuid is parsed; empty parentUuid normalises to null (T-263)', () {
+      final withParent = envelope(
+        type: 'user',
+        uuid: 'u8',
+        parentUuid: 'msg-A',
+        message: {'role': 'user', 'content': 'a sidechain prompt'},
+      );
+      final withoutParent = envelope(
+        type: 'user',
+        uuid: 'u9',
+        message: {'role': 'user', 'content': 'top-level'}, // parentUuid defaults to ''
+      );
+      final items = parseAll([withParent, withoutParent]);
+      expect(items[0].parentUuid, 'msg-A');
+      expect(items[1].parentUuid, isNull); // '' → null
+    });
   });
 
   // -------------------------------------------------------------------------

test/goldens/conversation_card_goldens_test.dart

@@ -51,6 +51,25 @@ void main() {
             ),
           ),
         ),
+        GoldenTestScenario(
+          name: 'agent / expanded (call → prompt → result layering, T-263)',
+          child: _wrap(
+            f,
+            const ConversationCard(
+              variant: ConversationCardVariant.bordered,
+              accent: Color(0xFF4C9AFF),
+              label: 'Task',
+              status: ConversationCardStatus.success,
+              collapsible: true,
+              collapsedByDefault: false,
+              body: Text('{ "description": "explore the codebase" }', textDirection: TextDirection.ltr),
+              extraSegments: [
+                CardSegment(label: 'prompt', child: Text('find all the widgets and summarise', textDirection: TextDirection.ltr)),
+                CardSegment(label: 'result', child: Text('found 42 widgets', textDirection: TextDirection.ltr)),
+              ],
+            ),
+          ),
+        ),
         GoldenTestScenario(
           name: 'error / header mark (call card stays separate from the red result card)',
           child: _wrap(