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/main.dart

@@ -368,6 +368,14 @@ Future<void> main() async {
         final file = File(path.startsWith('/') ? path : '${workRoot.path}/$path');
         return file.existsSync() ? file.absolute.path : null;
       },
+      readFile: (path) async {
+        final file = File(path.startsWith('/') ? path : '${workRoot.path}/$path');
+        try {
+          return file.existsSync() ? await file.readAsString() : null;
+        } catch (_) {
+          return null;
+        }
+      },
     );
     // `clide draw --file <doc>` — drive a drawing card into the Claude
     // conversation (T-318, drive-half of D-6). Reads the JSON doc relative to