feat: Add Claude Code plugin and marketplace support

Implement full Claude Code plugin structure with marketplace support,
enabling easy installation via /plugin commands.

## Changes

### Plugin Structure (Standard Compliant)

**Created `.claude-plugin/` directory with:**
- `plugin.json` - Moved from root, updated to standard schema
- `marketplace.json` - NEW: Enables marketplace discovery

**plugin.json changes:**
- ❌ Removed non-standard `skills` array (auto-discovered from skills/ directory)
- ❌ Removed non-standard `requires` field
- ✅ Updated `author` from string to object with name and url
- ✅ Added `homepage` field
- ✅ Follows official Claude Code plugin schema

### Installation Methods

**Method 1 - Direct Plugin Install** (Recommended):
```bash
/plugin install czlonkowski/n8n-skills
```

**Method 2 - Marketplace**:
```bash
/plugin marketplace add czlonkowski/n8n-skills
/plugin install  # Then select from list
```

**Method 3 - Manual**: Clone and copy (unchanged)

### Repository Structure

```
n8n-skills/
├── .claude-plugin/          # NEW - Plugin metadata
│   ├── plugin.json          # Standard schema
│   └── marketplace.json     # Marketplace listing
├── skills/                  # Auto-discovered by Claude Code
│   ├── n8n-expression-syntax/
│   ├── n8n-mcp-tools-expert/
│   ├── n8n-workflow-patterns/
│   ├── n8n-validation-expert/
│   └── n8n-node-configuration/
└── dist/
    └── n8n-mcp-skills-claude-code-v1.0.0.zip  # Updated with .claude-plugin/
```

### Benefits

✅ **One-command installation**: `/plugin install czlonkowski/n8n-skills`
✅ **Marketplace ready**: Can be browsed via marketplace UI
✅ **Auto-discovery**: Skills automatically detected from skills/ directory
✅ **Standard compliant**: Follows official Claude Code plugin specification
✅ **Dual distribution**: Works as both plugin and marketplace

### Documentation

Updated README.md with three installation methods:
1. Plugin install (recommended)
2. Marketplace browse and install
3. Manual installation (backward compatible)

Distribution package regenerated with .claude-plugin/ directory.

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

Co-Authored-By: Claude <noreply@anthropic.com>
Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

.claude-plugin/marketplace.json

@@ -0,0 +1,33 @@
+{
+  "name": "n8n-mcp-skills",
+  "description": "Expert skills for building n8n workflows",
+  "owner": {
+    "name": "Romuald Członkowski",
+    "url": "https://www.aiadvisors.pl/en"
+  },
+  "plugins": [
+    {
+      "name": "n8n-mcp-skills",
+      "source": ".",
+      "description": "Complete bundle: 5 expert skills for building flawless n8n workflows using n8n-mcp MCP server. Includes skills for expression syntax, MCP tools usage, workflow patterns, validation, and node configuration.",
+      "version": "1.0.0",
+      "author": {
+        "name": "Romuald Członkowski",
+        "url": "https://www.aiadvisors.pl/en"
+      },
+      "category": "automation",
+      "keywords": [
+        "n8n",
+        "workflow",
+        "mcp",
+        "automation",
+        "validation",
+        "expressions",
+        "skills"
+      ],
+      "homepage": "https://github.com/czlonkowski/n8n-skills",
+      "repository": "https://github.com/czlonkowski/n8n-skills",
+      "license": "MIT"
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -2,18 +2,11 @@
   "name": "n8n-mcp-skills",
   "version": "1.0.0",
   "description": "Expert skills for building n8n workflows with n8n-mcp",
-  "author": "Romuald Członkowski (https://www.aiadvisors.pl/en)",
+  "author": {
-  "license": "MIT",
+    "name": "Romuald Członkowski",
-  "skills": [
+    "url": "https://www.aiadvisors.pl/en"
-    "skills/n8n-expression-syntax",
-    "skills/n8n-mcp-tools-expert",
-    "skills/n8n-workflow-patterns",
-    "skills/n8n-validation-expert",
-    "skills/n8n-node-configuration"
-  ],
-  "requires": {
-    "mcp_servers": ["n8n-mcp"]
   },
+  "license": "MIT",
   "keywords": [
     "n8n",
     "workflow",
@@ -25,5 +18,6 @@
   "repository": {
     "type": "git",
     "url": "https://github.com/czlonkowski/n8n-skills"
-  }
+  },
+  "homepage": "https://github.com/czlonkowski/n8n-skills"
 }

README.md

@@ -102,6 +102,23 @@ Operation-aware node configuration guidance.
 
 ### Claude Code
 
+**Method 1: Plugin Installation** (Recommended)
+```bash
+# Install directly as a Claude Code plugin
+/plugin install czlonkowski/n8n-skills
+```
+
+**Method 2: Via Marketplace**
+```bash
+# Add as marketplace, then browse and install
+/plugin marketplace add czlonkowski/n8n-skills
+
+# Then browse available plugins
+/plugin install
+# Select "n8n-mcp-skills" from the list
+```
+
+**Method 3: Manual Installation**
 ```bash
 # 1. Clone this repository
 git clone https://github.com/czlonkowski/n8n-skills.git