fix: use exact match for win32 platform detection

The previous check used includes("win") which incorrectly matched
"darwin" (macOS) because it contains "win". This caused cmd.exe to be
used on macOS instead of /bin/sh.

src/infra/node-shell.ts

@@ -2,7 +2,7 @@ export function buildNodeShellCommand(command: string, platform?: string | null)
   const normalized = String(platform ?? "")
     .trim()
     .toLowerCase();
-  if (normalized.includes("win")) {
+  if (normalized === "win32") {
     return ["cmd.exe", "/d", "/s", "/c", command];
   }
   return ["/bin/sh", "-lc", command];