Add shadcn customization, evaluation, and rules documentation

- Create customization.md for theming and CSS variable usage.
- Add evals.json for testing shadcn/ui component prompts and expectations.
- Introduce mcp.md for MCP server setup and configuration.
- Document API differences between base and radix in base-vs-radix.md.
- Establish component composition rules in composition.md.
- Define forms and input handling guidelines in forms.md.
- Set icon usage standards in icons.md.
- Outline styling and customization best practices in styling.md.
- Update skills-lock.json to include shadcn source information.

skills-lock.json

@@ -5,6 +5,11 @@
       "source": "github/awesome-copilot",
       "sourceType": "github",
       "computedHash": "2621a44fbd9fc2636953d1e6e39e5faeed995f7fb958ec12cc98a2f0576f6fa7"
+    },
+    "shadcn": {
+      "source": "shadcn/ui",
+      "sourceType": "github",
+      "computedHash": "642a177bee320618caa49f5106cadb4e7594c606e867f61ba7b56d19cf745cd5"
     }
   }
 }