feat(image): --file metadata payload for annotated image cards (T-316)

`clide image show --file meta.json` reads a {path,label,description,caption}
payload so an image can carry a title and a longer description, not just a
one-line caption. ImageMessage + the image card render the richer metadata;
the bare `image show <path> [--caption]` form is unchanged. Honest userError
on a malformed/missing payload. Text annotation only (option a) — visual
marker overlays stay a follow-up.

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

lib/builtin/claude/src/conversation_view.dart

@@ -749,6 +749,11 @@ class _ConversationTurn extends StatelessWidget {
       body: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
+          // Annotation title above the image (T-316), when a --file payload set it.
+          if (m.label != null && m.label!.isNotEmpty) ...[
+            ClideText(m.label!, fontSize: clideFontMeta, fontWeight: FontWeight.w600, color: tokens.globalForeground),
+            const SizedBox(height: 4),
+          ],
           // The card stays display-only (D-78); the click is a navigation
           // gesture that opens the full-screen lightbox (T-252), not an inline
           // control.
@@ -770,6 +775,10 @@ class _ConversationTurn extends StatelessWidget {
               ),
             ),
           ),
+          if (m.description != null && m.description!.isNotEmpty) ...[
+            const SizedBox(height: 6),
+            ClideText(m.description!, fontSize: clideFontCaption, color: tokens.globalTextMuted),
+          ],
           if (caption != null && caption.isNotEmpty) ...[const SizedBox(height: 4), ClideText(caption, fontSize: clideFontMeta, color: tokens.globalTextMuted)],
         ],
       ),

lib/builtin/claude/src/extension.dart

@@ -641,6 +641,8 @@ class ClaudeExtension extends ClideExtension {
         isSidechain: false,
         path: path,
         caption: m.data['caption'] as String?,
+        label: m.data['label'] as String?,
+        description: m.data['description'] as String?,
       ),
     );
   }

lib/builtin/claude/src/transcript_reader.dart

@@ -178,7 +178,15 @@ final class AssistantToolUse extends ConversationItem {
 /// driver has already resolved (workspace-relative paths are resolved before
 /// injection); [caption] is an optional one-line label.
 final class ImageMessage extends ConversationItem {
-  const ImageMessage({required super.uuid, required super.timestamp, required super.isSidechain, required this.path, this.caption});
+  const ImageMessage({
+    required super.uuid,
+    required super.timestamp,
+    required super.isSidechain,
+    required this.path,
+    this.caption,
+    this.label,
+    this.description,
+  });
 
   /// Absolute path to the image file on disk.
   final String path;
@@ -186,6 +194,10 @@ final class ImageMessage extends ConversationItem {
   /// Optional caption shown under the image.
   final String? caption;
 
+  /// Optional richer annotations from a `--file` metadata payload (T-316): a
+  /// title/label above the image and a longer description beneath it.
+  final String? label, description;
+
   @override
   String toString() => 'ImageMessage($path${caption != null ? ', "$caption"' : ''})';
 }