chore: Remove dss-claude-plugin directory

Removing obsolete plugin directory after consolidation.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dss-claude-plugin/core/skins/classic.json

@@ -1,21 +0,0 @@
-{
-  "meta": {
-    "id": "classic",
-    "version": "2.0.0",
-    "parent": "base"
-  },
-  "tokens": {
-    "colors": {
-      "primary": "#3B82F6",
-      "secondary": "#10B981",
-      "danger": "#EF4444",
-      "background": "#F3F4F6",
-      "surface": "#FFFFFF",
-      "text": "#1F2937"
-    },
-    "borderRadius": {
-      "default": "0.25rem",
-      "lg": "0.5rem"
-    }
-  }
-}