fix: correct search_files result field name in formatToolResultForModel

The search_files tool handler returns results in a 'results' field
(containing per-file match arrays), but formatToolResultForModel was
referencing 'result.matches' which is undefined. This caused the model
to never see actual search file results, breaking the search → read
tool chain workflow.

src/renderer/services/agent-engine.ts

@@ -328,7 +328,7 @@ function formatToolResultForModel(toolName: string, result: ToolResult): string
         query: result.query,
         total_matches: result.total_matches,
         total_files: result.total_files,
-        matches: result.matches
+        results: result.results
       });
     }