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/plugin.json

@@ -0,0 +1,23 @@
+{
+  "name": "n8n-mcp-skills",
+  "version": "1.0.0",
+  "description": "Expert skills for building n8n workflows with n8n-mcp",
+  "author": {
+    "name": "Romuald Członkowski",
+    "url": "https://www.aiadvisors.pl/en"
+  },
+  "license": "MIT",
+  "keywords": [
+    "n8n",
+    "workflow",
+    "automation",
+    "mcp",
+    "validation",
+    "expressions"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/czlonkowski/n8n-skills"
+  },
+  "homepage": "https://github.com/czlonkowski/n8n-skills"
+}