feat: implement comprehensive plugin architecture and extensions system (issue #19)
Complete plugin system implementation providing extensible architecture for MarkiTect:
🏗️ **Core Plugin Architecture**:
- BasePlugin abstract class with lifecycle management (initialize/cleanup)
- Specialized plugin types: ProcessorPlugin, FormatterPlugin, ValidatorPlugin, ExporterPlugin, CommandPlugin
- PluginMetadata system with version, dependencies, and type information
- Plugin initialization and configuration validation
🔍 **Plugin Discovery & Management**:
- PluginManager with automatic discovery from built-in modules and directories
- PluginRegistry for centralized plugin registration and lifecycle management
- Support for plugin loading, unloading, and reloading with configuration
- Plugin discovery from multiple sources (built-in, directories, packages)
🛠️ **CLI Integration**:
- markitect plugin-list: List all available plugins with metadata
- markitect plugin-load: Load plugins with optional configuration
- markitect plugin-unload: Unload plugins and cleanup resources
- markitect plugin-info: Show detailed plugin information
- markitect plugin-discover: Discover and refresh plugin catalog
📦 **Built-in Plugins**:
- JSON/YAML/Table formatters for output formatting
- Markdown/Text processors for content processing
- Auto-registered via @register_plugin decorator
- Comprehensive configuration options
🔧 **Developer Experience**:
- @register_plugin decorator for easy plugin registration
- Plugin configuration validation and error handling
- Comprehensive API documentation with examples
- Plugin development guide and best practices
📋 **Example Plugins**:
- Advanced text processor with case conversion and pattern replacement
- XML/CSV formatters demonstrating custom output formats
- Complete examples showing plugin development patterns
🧪 **Test Coverage**:
- 59 comprehensive tests covering all plugin functionality
- Tests for plugin lifecycle, registration, discovery, and CLI integration
- Error handling and edge case coverage
- Built-in plugin validation
Technical Implementation:
- Plugin types: processor, formatter, validator, exporter, generator, importer, transformer, extension, backend, command
- Configuration-driven plugin management with YAML/JSON support
- Graceful error handling and plugin isolation
- Plugin dependency validation and compatibility checking
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
tegwick
b0de32d083