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/ROADMAP.md

@@ -17,7 +17,6 @@ A parallel workstream (not a numbered chunk) establishing the plugin distributio
 
 **Phase 1 — marketplace scaffold and kyberforge plugin** ✅ complete (2026-06-20)
 - `.claude-plugin/marketplace.json` and `.github/plugin/marketplace.json` — dual-path marketplace manifest (Claude Code + Copilot CLI)
-- `plugins/hello-world/` — demo plugin validating scaffolding
 - `plugins/kyberforge/` — marketplace management toolkit: `create-plugin`, `marketplace-architect`, `write-skill`, `write-eval` skills, bundled template (`assets/plugin-template/`), reference docs, scripts, and evals
 - `templates/plugin/` removed — bundled into `kyberforge` plugin; `docs/research/plugin-marketplace-architecture.md` moved into `plugins/kyberforge/docs/`
 - Skills `write-eval`, `write-skill`, `create-plugin`, `marketplace-architect` removed from `.agents/skills/` — now only available via `kyberforge` plugin install

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