feat: Add plugin-dev toolkit for comprehensive plugin development

Adds the plugin-dev plugin to public marketplace. A comprehensive toolkit for
developing Claude Code plugins with 7 expert skills, 3 AI-assisted agents, and
extensive documentation covering the complete plugin development lifecycle.

Key features:
- 7 skills: hook-development, mcp-integration, plugin-structure, plugin-settings,
  command-development, agent-development, skill-development
- 3 agents: agent-creator (AI-assisted generation), plugin-validator (structure
  validation), skill-reviewer (quality review)
- 1 command: /plugin-dev:create-plugin (guided 8-phase workflow)
- 10 utility scripts for validation and testing
- 21 reference docs with deep-dive guidance (~11k words)
- 9 working examples demonstrating best practices

Changes for public release:
- Replaced all references to internal repositories with "Claude Code"
- Updated MCP examples: internal.company.com → api.example.com
- Updated token variables: ${INTERNAL_TOKEN} → ${API_TOKEN}
- Reframed agent-creation-system-prompt as "proven in production"
- Preserved all ${CLAUDE_PLUGIN_ROOT} references (186 total)
- Preserved valuable test blocks in core modules

Validation:
- All 3 agents validated successfully with validate-agent.sh
- All JSON files validated with jq
- Zero internal references remaining
- 59 files migrated, 21,971 lines added

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

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

plugins/plugin-dev/skills/mcp-integration/examples/stdio-server.json

@@ -0,0 +1,26 @@
+{
+  "_comment": "Example stdio MCP server configuration for local file system access",
+  "filesystem": {
+    "command": "npx",
+    "args": ["-y", "@modelcontextprotocol/server-filesystem", "${CLAUDE_PROJECT_DIR}"],
+    "env": {
+      "LOG_LEVEL": "info"
+    }
+  },
+  "database": {
+    "command": "${CLAUDE_PLUGIN_ROOT}/servers/db-server.js",
+    "args": ["--config", "${CLAUDE_PLUGIN_ROOT}/config/db.json"],
+    "env": {
+      "DATABASE_URL": "${DATABASE_URL}",
+      "DB_POOL_SIZE": "10"
+    }
+  },
+  "custom-tools": {
+    "command": "python",
+    "args": ["-m", "my_mcp_server", "--port", "8080"],
+    "env": {
+      "API_KEY": "${CUSTOM_API_KEY}",
+      "DEBUG": "false"
+    }
+  }
+}