feat: Convert to Claude Code plugin marketplace

Transform repository into a plugin marketplace structure with two plugins:

- claude-code plugin: Complete toolkit with 5 skills
  * claude-code-plugins
  * claude-code-slash-commands
  * claude-code-hooks
  * claude-code-subagents
  * claude-code-memory

- claude-skills plugin: Meta-skill for creating Agent Skills
  * Comprehensive best practices guide
  * Templates and examples
  * Progressive disclosure patterns

Infrastructure:
- Add marketplace.json manifest
- Create plugin.json for each plugin
- Update documentation for marketplace structure
- Add contribution and testing guides

Installation:
- /plugin install claude-code@claude-skills
- /plugin install claude-skills@claude-skills

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.claude-plugin/marketplace.json

@@ -0,0 +1,19 @@
+{
+  "name": "claude-skills-marketplace",
+  "owner": {
+    "name": "Claude Skills Contributors"
+  },
+  "description": "A curated collection of Agent Skills and plugins for Claude Code and Claude.ai",
+  "plugins": [
+    {
+      "name": "claude-code",
+      "source": "./claude-code",
+      "description": "Complete toolkit for Claude Code: plugins, slash commands, hooks, subagents, and memory management"
+    },
+    {
+      "name": "claude-skills",
+      "source": "./claude-skills-plugin",
+      "description": "Meta-skill for creating effective Agent Skills with best practices and templates"
+    }
+  ]
+}