feat: implement lightweight full text search plugin using SQLite FTS5 (issue #83)

Added comprehensive full text search capabilities as a lightweight plugin.

Key features:
- SQLite FTS5-based search engine with no external dependencies
- Automatic indexing via database triggers for real-time updates
- Advanced query support: phrase search, boolean operators, proximity search
- Complete CLI interface with search commands
- Graceful fallback to LIKE queries when FTS5 unavailable
- Plugin architecture integration for extensibility

CLI Commands:
- `markitect search init` - Initialize search indexes
- `markitect search query` - Perform full text searches
- `markitect search status` - View index statistics
- `markitect search rebuild` - Rebuild indexes from scratch

Search Features:
- Content type filtering (files, schemas, all)
- Result pagination and formatting options
- Query validation and syntax assistance
- Performance optimization and index maintenance

Technical Implementation:
- FTSSearchPlugin: Main search plugin class
- SearchIndexer: FTS5 table management and indexing
- QueryParser: Query optimization and FTS5 syntax conversion
- Comprehensive error handling and fallback mechanisms
- 25 test cases covering all functionality

Documentation includes complete usage guide and examples.

Resolves issue #83: Full text search

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

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

markitect/plugins/builtin/search/__init__.py

@@ -0,0 +1,12 @@
+"""
+Full text search plugin for MarkiTect using SQLite FTS5.
+
+Provides lightweight, high-performance full text search capabilities
+as a plugin to the MarkiTect system.
+"""
+
+from .fts_search import FTSSearchPlugin
+from .indexer import SearchIndexer
+from .query_parser import QueryParser
+
+__all__ = ['FTSSearchPlugin', 'SearchIndexer', 'QueryParser']