chore: remove hello-world demo plugin

No longer needed — kyberforge plugin-create provides a bundled template that
serves as the canonical scaffold reference. Remove hello-world from marketplace
manifests and update docs accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/plugin/marketplace.json

@@ -4,11 +4,6 @@
   "description": "AI development skills for Claude Code and GitHub Copilot CLI — factory, design, implement, review, and cross-cutting workflows.",
   "version": "0.1.0",
   "plugins": [
-    {
-      "name": "hello-world",
-      "description": "A minimal example plugin to validate marketplace scaffolding.",
-      "source": "./plugins/hello-world"
-    },
     {
       "name": "kyberforge",
       "description": "Skills and agents for creating, maintaining, and managing a Claude Code / Copilot CLI plugin marketplace.",