fix: properly format agentDone tool completion message

The agentDone tool was displaying "[object Object]" in its completion message
instead of the actual result string. This was because it was trying to
interpolate the entire output object instead of just the result property.

This change updates the logReturns function to correctly display the result
property from the output object.

Author: bhouston

packages/agent/src/tools/agent/agentDone.ts

@@ -27,6 +27,6 @@ export const agentDoneTool: Tool<Parameters, ReturnType> = {
   execute: ({ result }) => Promise.resolve({ result }),
   logParameters: () => {},
   logReturns: (output, { logger }) => {
-    logger.log(`Completed: ${output}`);
+    logger.log(`Completed: ${output.result}`);
   },
 };