fix(cli): hide detached Windows spawns

[kvnloo]

Summary

• pass windowsHide: true to telemetry exit flush, custom browser launch, and background preview detached spawns
• cover each real spawn path with a behavior regression test
• match the existing auto-update spawn behavior

Fixes #3476

Verification

• bun run --filter @hyperframes/cli test -- src/telemetry/client.test.ts src/utils/openBrowser.test.ts src/commands/previewLifecycle.test.ts (61 passed)
• bun run --filter @hyperframes/cli typecheck
• bun run lint
• bun run format:check
• bun run build
• git diff --check
• sabotage check: removing windowsHide from openBrowser made its new regression test fail

The full bun run test is not clean in this environment. Studio has 97 failures caused primarily by unavailable localStorage, and the full CLI run has 3 unrelated skillsManifest.test.ts lock-pruning failures. The focused tests for all changed paths pass.

Windows real-device validation has not been performed.

Scope

This does not change ffmpeg handling from #3379 or chrome-headless-shell handling from #3430. It does not add a shared spawn helper or lint rule.

AI assistance

Implemented and verified with Hermes Agent using Codex. I reviewed the resulting diff and test evidence.