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/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(