Add shadcn registry documentation and component rules

- Created registry.md for shadcn registry authoring and address schemes.
- Added base-vs-radix.md to outline API differences between base and radix components.
- Introduced chat.md for chat and messaging component guidelines.
- Established composition.md for component composition best practices.
- Defined forms.md for forms and input handling rules.
- Created icons.md for icon usage and guidelines.
- Added styling.md for styling and customization best practices.
- Initialized skills-lock.json to track shadcn skill configuration.

skills-lock.json

@@ -0,0 +1,11 @@
+{
+  "version": 1,
+  "skills": {
+    "shadcn": {
+      "source": "shadcn/ui",
+      "sourceType": "github",
+      "skillPath": "skills/shadcn/SKILL.md",
+      "computedHash": "d81caa0f86aabab65b25e302d454f23a3328760386ed9078345584c0d5c8058e"
+    }
+  }
+}