fix: add shell: true on Windows so spawnSync can resolve .cmd shims (#13)

On Windows, Node.js `spawnSync` without `shell: true` uses
`CreateProcess`, which only resolves `.exe` files. npm installs global
tools (like `codex`) as `.cmd` shims, so `spawnSync("codex", ...)`
returns ENOENT even when codex is correctly installed and on PATH.

Adding `shell: process.platform === "win32"` routes through `cmd.exe`
on Windows, which properly resolves `.cmd`, `.bat`, and PATHEXT entries.
No behavior change on macOS/Linux since the condition evaluates to false.

plugins/codex/scripts/lib/process.mjs

@@ -7,7 +7,8 @@ export function runCommand(command, args = [], options = {}) {
     env: options.env,
     encoding: "utf8",
     input: options.input,
-    stdio: options.stdio ?? "pipe"
+    stdio: options.stdio ?? "pipe",
+    shell: process.platform === "win32"
   });
 
   return {