feat: disable PostHog telemetry in CI/mock environments 🛑

**✨ Features**
- Add an offlineMode flag to Node and Web PostHog adapters that disables initialization when running in CI or when POSTHOG_MOCK is set, preventing outgoing telemetry.

**🐛 Fixes**
- Guard flush and shutdown to no-op when the adapter is disabled or the client is missing to avoid calling methods on undefined objects.

**🧹 Chores**
- Enable POSTHOG_MOCK in the CI workflow and in package.json test scripts so test runs do not send telemetry.

**✅ Tests**
- Update tests to save/restore CI/POSTHOG_MOCK env vars, stub additional PostHog methods, and add assertions that reloadFeatureFlags is a no-op when disabled and that flush/shutdown delegate when enabled.