fix(config): use r+ mode for fsync on Windows to prevent migration retry loop

openSync with read-only mode fails fsync on Windows because FlushFileBuffers requires write-permission FD. This caused atomic writes to fail silently, leaving migrated config unwritten and triggering repeated migration + .bak.<timestamp> generation on every startup.

Same root cause as PR #3644 (#3643). Hyperplan disappear is a secondary symptom of plugin load instability.

Fixes #3877

src/shared/write-file-atomically.ts

@@ -16,7 +16,7 @@ export function writeFileAtomically(
 ): void {
   const tempPath = `${filePath}.tmp`
   writeFileSync(tempPath, content, "utf-8")
-  const tempFileDescriptor = openSync(tempPath, "r")
+  const tempFileDescriptor = openSync(tempPath, "r+")
   try {
     tolerantFsyncSync(tempFileDescriptor, `writeFileAtomically:${filePath}`, deps.fsyncSync)
   } finally {