fold sub-agent prompts via parent_tool_use_id (T-338)

In live stream-json sessions a sub-agent's spawning prompt is tagged
with parent_tool_use_id, not the transcript JSONL's isSidechain +
parentUuid. The parser ignored that field, so the prompt parsed as a
main-thread user turn and rendered as a blue "you" card above the
Activity Agent card instead of folding into it.

Carry parent_tool_use_id onto ConversationItem; its presence now marks
the item as a sidechain message. The sidechain fold resolves ownership
directly by tool-use id (no transcript-only uuid chain to walk), so the
prompt folds into its Agent card and the run nests under it as before.

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

test/builtin/claude/transcript_reader_test.dart

@@ -378,6 +378,30 @@ void main() {
       expect(items[0].parentUuid, 'msg-A');
       expect(items[1].parentUuid, isNull); // '' → null
     });
+
+    test('stream-json parent_tool_use_id marks a sidechain message (T-338)', () {
+      // The stream-json wire tags sub-agent messages with parent_tool_use_id and
+      // NO isSidechain flag — we must treat it as a sidechain item anyway.
+      final raw = envelope(
+        type: 'user',
+        uuid: 'sp',
+        message: {'role': 'user', 'content': 'go explore the codebase'},
+      )..['parent_tool_use_id'] = 'toolu_task1';
+      final items = parseAll([raw]);
+      expect(items.first.isSidechain, isTrue);
+      expect(items.first.parentToolUseId, 'toolu_task1');
+    });
+
+    test('empty parent_tool_use_id normalises to null and stays main-thread (T-338)', () {
+      final raw = envelope(
+        type: 'user',
+        uuid: 'mt',
+        message: {'role': 'user', 'content': 'a normal turn'},
+      )..['parent_tool_use_id'] = '';
+      final items = parseAll([raw]);
+      expect(items.first.isSidechain, isFalse);
+      expect(items.first.parentToolUseId, isNull);
+    });
   });
 
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