feat: sunbeam reindex-code CLI verb + ReindexCode proto

Proto: ReindexCode RPC with org/repo/branch filters.
CLI: sunbeam reindex-code [--org studio] [--repo owner/name] [--endpoint ...]
Calls Sol's gRPC ReindexCode endpoint, prints indexed symbol count.

sunbeam/tests/code_integration.rs

@@ -72,6 +72,10 @@ impl CodeAgent for MockCodeAgent {
 
         Ok(Response::new(Box::pin(ReceiverStream::new(rx))))
     }
+
+    async fn reindex_code(&self, _req: Request<ReindexCodeRequest>) -> Result<Response<ReindexCodeResponse>, Status> {
+        Ok(Response::new(ReindexCodeResponse { repos_indexed: 0, symbols_indexed: 0, error: "mock".into() }))
+    }
 }
 
 #[tokio::test]