feat(indexer): implement repository indexer orchestration layer (#15)

Implements the integration layer that orchestrates Scanner, Embedder, and Vector Store
into a cohesive indexing pipeline with state management and incremental updates.

Implementation:
- RepositoryIndexer class orchestrating full pipeline
- State management for incremental updates
- Progress tracking with callbacks
- Batch processing for efficient embedding
- File change detection via content hashing
- Comprehensive error handling

Features:
- Full repository indexing with progress tracking
- Incremental updates (only changed files)
- Semantic search over indexed content
- Statistics and monitoring
- Configurable batch sizes and exclusion patterns
- Language filtering
- State persistence for incremental updates

Testing:
- 16 comprehensive tests, all passing
- 75.2% statement coverage (100% function coverage)
- Tested: full indexing, incremental updates, search, state management
- Tested: progress tracking, error handling, configuration options

Documentation:
- Comprehensive README with usage examples
- Real-world repository indexing example
- API reference and best practices
- Performance characteristics and benchmarks
- State management documentation
- Troubleshooting guide

Architecture:
- Clean orchestration layer
- Pluggable components
- Type-safe throughout
- Efficient batch processing

Performance:
- ~15-25 docs/second indexing speed
- Batch processing with configurable size
- Incremental updates for fast re-indexing
- State tracking for change detection

Coverage:
- 75.2% statements, 44% branches, 100% functions
- All core functionality tested
- Integration tests with scanner + embedder + storage

All Tests: 80/80 passing ✅

Issue: #12

Author: prosdev