Fix race condition causing double JSON output on Windows

The unawaited finishWithMessage calls allowed execution to fall through
to outputDeps, producing two JSON objects on stdout when the async exit
was delayed by the Windows flush workaround.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: silverwind

index.ts

@@ -1810,7 +1810,7 @@ async function main(): Promise<void> {
     }
   }
   if (numDependencies === 0) {
-    finishWithMessage("No dependencies found, nothing to do.");
+    return finishWithMessage("No dependencies found, nothing to do.");
   }
 
   let numEntries = 0;
@@ -1819,7 +1819,7 @@ async function main(): Promise<void> {
   }
 
   if (!numEntries) {
-    finishWithMessage("All dependencies are up to date.");
+    return finishWithMessage("All dependencies are up to date.");
   }
 
   const exitCode = outputDeps(deps);