fix(config): use r+ when fsyncing migrated temp file

Apply the Windows-safe open mode in migrateLegacyPluginEntry and add a regression test to assert the temp fd is opened with r+ before fsync.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/migrate-legacy-plugin-entry.ts

@@ -54,7 +54,7 @@ export function migrateLegacyPluginEntry(configPath: string): boolean {
 
     const tempPath = `${configPath}.tmp`
     writeFileSync(tempPath, updated, "utf-8")
-    const tempFileDescriptor = openSync(tempPath, "r")
+    const tempFileDescriptor = openSync(tempPath, "r+")
     try {
       fsyncSync(tempFileDescriptor)
     } finally {