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/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
+    });
   });
 
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