fix: sanitize os.system() shell injection in manimgl scene templates

os.system(f"manimgl {__file__} ClassName") interpolates the script's own
path into a shell string. These templates are meant to be copied and
renamed per-scene by an agent, so a scene/folder name containing shell
metacharacters is a real injection path, not just malformed input.

Switched to subprocess.run() with an argument list (no shell=True), so
there's nothing left for a shell to interpret regardless of what the
path contains. Same fix applied in both duplicate locations
(.claude/skills and .agents/skills) since the files are identical.

Reviewed scripts/lib/tts.mjs's child_process usage as part of the same
report -- not included in this PR, it already passes args as a real
array with no shell:true anywhere in the call chain, so it isn't
actually exploitable.

.agents/skills/manimgl-best-practices/templates/3d_scene.py

@@ -254,5 +254,5 @@ class LightingTemplate(Scene):
 
 
 if __name__ == "__main__":
-    import os
-    os.system(f"manimgl {__file__} ThreeDSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "ThreeDSceneTemplate"])

.agents/skills/manimgl-best-practices/templates/basic_scene.py

@@ -130,5 +130,5 @@ class AnimationShowcase(Scene):
 if __name__ == "__main__":
     # This allows you to run: python basic_scene.py
     # (though using manimgl is recommended)
-    import os
-    os.system(f"manimgl {__file__} BasicSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "BasicSceneTemplate"])

.agents/skills/manimgl-best-practices/templates/math_scene.py

@@ -329,5 +329,5 @@ class MatrixTemplate(Scene):
 
 
 if __name__ == "__main__":
-    import os
-    os.system(f"manimgl {__file__} MathSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "MathSceneTemplate"])

.claude/skills/manimgl-best-practices/templates/3d_scene.py

@@ -254,5 +254,5 @@ class LightingTemplate(Scene):
 
 
 if __name__ == "__main__":
-    import os
-    os.system(f"manimgl {__file__} ThreeDSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "ThreeDSceneTemplate"])

.claude/skills/manimgl-best-practices/templates/basic_scene.py

@@ -130,5 +130,5 @@ class AnimationShowcase(Scene):
 if __name__ == "__main__":
     # This allows you to run: python basic_scene.py
     # (though using manimgl is recommended)
-    import os
-    os.system(f"manimgl {__file__} BasicSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "BasicSceneTemplate"])

.claude/skills/manimgl-best-practices/templates/math_scene.py

@@ -329,5 +329,5 @@ class MatrixTemplate(Scene):
 
 
 if __name__ == "__main__":
-    import os
-    os.system(f"manimgl {__file__} MathSceneTemplate")
+    import subprocess
+    subprocess.run(["manimgl", __file__, "MathSceneTemplate"])