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/plugin-structure/examples/minimal-plugin.md

@@ -0,0 +1,83 @@
+# Minimal Plugin Example
+
+A bare-bones plugin with a single command.
+
+## Directory Structure
+
+```
+hello-world/
+├── .claude-plugin/
+│   └── plugin.json
+└── commands/
+    └── hello.md
+```
+
+## File Contents
+
+### .claude-plugin/plugin.json
+
+```json
+{
+  "name": "hello-world"
+}
+```
+
+### commands/hello.md
+
+```markdown
+---
+name: hello
+description: Prints a friendly greeting message
+---
+
+# Hello Command
+
+Print a friendly greeting to the user.
+
+## Implementation
+
+Output the following message to the user:
+
+> Hello! This is a simple command from the hello-world plugin.
+>
+> Use this as a starting point for building more complex plugins.
+
+Include the current timestamp in the greeting to show the command executed successfully.
+```
+
+## Usage
+
+After installing the plugin:
+
+```
+$ claude
+> /hello
+Hello! This is a simple command from the hello-world plugin.
+
+Use this as a starting point for building more complex plugins.
+
+Executed at: 2025-01-15 14:30:22 UTC
+```
+
+## Key Points
+
+1. **Minimal manifest**: Only the required `name` field
+2. **Single command**: One markdown file in `commands/` directory
+3. **Auto-discovery**: Claude Code finds the command automatically
+4. **No dependencies**: No scripts, hooks, or external resources
+
+## When to Use This Pattern
+
+- Quick prototypes
+- Single-purpose utilities
+- Learning plugin development
+- Internal team tools with one specific function
+
+## Extending This Plugin
+
+To add more functionality:
+
+1. **Add commands**: Create more `.md` files in `commands/`
+2. **Add metadata**: Update `plugin.json` with version, description, author
+3. **Add agents**: Create `agents/` directory with agent definitions
+4. **Add hooks**: Create `hooks/hooks.json` for event handling