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>

docs/spec/architecture.md

@@ -31,7 +31,6 @@ ai-development/
 ├── .github/plugin/
 │   └── marketplace.json         # Mirror of .claude-plugin/marketplace.json for Copilot CLI canonical path
 ├── plugins/                     # Installable plugin units (each is a self-contained deployable)
-│   ├── hello-world/             # Demo plugin validating marketplace scaffolding
 │   └── kyberforge/              # Marketplace management toolkit (create-plugin, marketplace-architect, write-skill, write-eval)
 ├── core/                        # Provider-agnostic source of truth
 │   ├── AGENTS.md                # Global always-on rules (Communication + Behavior); deployed to ~/.agents/AGENTS.md