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-code/.claude-plugin/plugin.json

@@ -0,0 +1,17 @@
+{
+  "name": "claude-code",
+  "description": "Comprehensive skills for mastering Claude Code features: plugins, slash commands, hooks, subagents, and memory management",
+  "version": "1.0.0",
+  "author": {
+    "name": "Claude Skills Contributors"
+  },
+  "keywords": [
+    "claude-code",
+    "plugins",
+    "slash-commands",
+    "hooks",
+    "subagents",
+    "memory",
+    "productivity"
+  ]
+}