feat(srp): add explicit skybox draw primitive

- add ScriptableRenderContext.DrawSkybox as a public main-scene draw primitive
- route default RecordScene and DrawSkyboxPass recording through explicit draw calls
- expose the new DrawSkybox API in the scriptable render context probe

managed/GameScripts/ScriptableRenderContextApiSurfaceProbe.cs

@@ -14,6 +14,7 @@ namespace Gameplay
         public bool HasPublicContextDrawRenderersByType;
         public bool HasPublicContextDrawRenderersByDesc;
         public bool HasPublicContextDrawRenderersByDescAndSettings;
+        public bool HasPublicContextDrawSkybox;
         public bool HasPublicContextDrawOpaqueRenderers;
         public bool HasPublicContextDrawTransparentRenderers;
         public bool HasPublicContextRecordOpaqueScenePhase;
@@ -149,6 +150,10 @@ namespace Gameplay
                         typeof(DrawingSettings)
                     },
                     null) != null;
+            HasPublicContextDrawSkybox =
+                contextType.GetMethod(
+                    "DrawSkybox",
+                    PublicInstanceMethodFlags) != null;
             HasPublicContextDrawOpaqueRenderers =
                 contextType.GetMethod(
                     "DrawOpaqueRenderers",