docs: Drop scratch ADR reference from SkillMatrixSerializer docstring

The `per ADR-0011's convention` annotation pointed at a no-commit
decision record (scratch/decisions/, gitignored). The Read/Write
split convention is documented in the module docstring at the top
of `serializers.py`; dangling reference removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Nickatak

backend/ctj_api/serializers.py

@@ -133,8 +133,8 @@ class Meta:
 class SkillMatrixSerializer(serializers.ModelSerializer):
     """Read/write serializer for `SkillMatrix` records.
 
-    Note: defined but never imported. Not split into Read/Write per
-    ADR-0011's convention because the class is currently dead code.
+    Note: defined but never imported. Not split into Read/Write
+    because the class is currently dead code.
     The cleanup PR drops it. If `SkillMatrix` becomes API-exposed
     later, replace this with `SkillMatrixReadSerializer` (and
     `SkillMatrixWriteSerializer` if a write endpoint is added).