fix: hide detached broker window on Windows

[e345ee]

Summary

• Add windowsHide: true to the detached broker spawn path.
• Add a focused unit test covering the broker spawn options.

Fixes #440.

Test plan

• node --test tests/broker-lifecycle.test.mjs
• git diff --check
• npm test
• npm run build

[Wintersta7e]

The spawnImpl seam plus asserting the captured options is a better test than scanning source text — it proves the option actually reaches spawn, rather than that the string appears somewhere in the file.

Worth folding in while you're in here: there's a third spawn site with the same problem that neither #440 nor #441 covers. plugins/codex/scripts/stop-review-gate-hook.mjs spawns the companion with

spawnSync(process.execPath, [scriptPath, "task", "--json", prompt], {
  cwd, env: childEnv, encoding: "utf8", timeout: STOP_REVIEW_TIMEOUT_MS
});

and no windowsHide. Unlike the broker, that one runs at every turn end while the review gate is enabled, so on Windows it's a recurring console window rather than a one-off. lib/process.mjs already sets windowsHide: true on its own spawnSync, which makes the hook the odd one out.