refactor(rendering): retire legacy SkyboxPass

[JeanPhilippeKernel]

Closes #809

Summary

• Replace the legacy SkyboxPass and skybox shaders with EnvironmentBackgroundPass.
• Preserve HDRI and fixed fallback presentation while using the camera far depth for reverse-Z.
• Remove obsolete references, refresh architecture docs, and add focused coverage.

Validation

• Debug: ZEngineTests full suite (four existing Vulkan-dependent skips)
• Debug: 42 focused sky/environment tests
• Release: ZEngineTests and Obelisk build; 42 focused sky/environment tests
• Obelisk Debug shader package builds with environment_background shaders
• Repository search confirms no SkyboxPass/skybox references remain