[Docs] (tools/cts): add the desktop Linux CTS skill

The Android and Windows paths each have a skill; the desktop Linux one had only
a runner script and a README section, so it was the least discoverable of the
three despite being the one to reach for while iterating - it needs no device
and no GPU, and a single test group takes seconds rather than hours.

Records what the other two skills cannot: that the toolchain has to be GCC 13+
or Clang 20+ (Clang 18 reports __cpp_concepts as 201907L, which switches
libstdc++'s <expected> off and breaks the shader transpiler), that
EGL_PLATFORM=surfaceless is mandatory for DirectGLES and why the symptom points
at the wrong call, and which of this environment's results are MobileGL's own
versus artefacts of software rendering.

Also states the rule the other skills only imply: report Espryt and Magma
separately. They fail different cases, and one combined number hides which
backend a change moved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DnW6wQYv2Pvrfisqak8UJu

tools/cts/README.md

@@ -9,7 +9,8 @@ This directory contains three supported paths:
   `skills/wgl-gl-cts-on-mobilegl/SKILL.md`.
 - Desktop Linux x64 / MobileGL EGL: `scripts/run_cts_local.py` against the
   `mobilegl-desktop` VK-GL-CTS target, documented in "Desktop Linux workflow"
-  below. This is the path that needs no device and no GPU.
+  below and in `skills/linux-gl-cts-on-mobilegl/SKILL.md`. This is the path that
+  needs no device and no GPU.
 
 Windows prerequisites are Git, Python 3.9+, CMake, Visual Studio 2022's Desktop
 C++ workload, and a Vulkan SDK visible to CMake. DirectVulkan also needs a