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/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"' : ''})';
 }