Create document formatting utilities for enhanced embedding quality:
- formatDocumentText(): Combine type, name, and content for better semantic search
- formatDocumentTextWithSignature(): Include function signatures for enhanced searchability
- truncateText(): Limit text length while preserving important content
- cleanDocumentText(): Normalize whitespace and reduce token count

These utilities are independent and provide flexible text formatting
options for optimizing embedding generation and search relevance.

Added 27 comprehensive tests including:
- Basic formatting with name/text/signature combinations
- Empty and edge case handling
- Multiline text preservation
- Truncation with ellipsis (various lengths)
- Whitespace normalization (spaces, newlines, tabs)
- Integration scenarios (format → clean → truncate pipeline)

100% coverage on all formatting utilities.

Create document transformation utilities for embedding pipeline:
- prepareDocumentsForEmbedding(): Transform Document[] → EmbeddingDocument[]
- prepareDocumentForEmbedding(): Single document variant for incremental indexing
- filterDocumentsByExport(): Filter by public/private API
- filterDocumentsByType(): Filter by document type (function, class, etc.)
- filterDocumentsByLanguage(): Case-insensitive language filtering

These utilities depend on formatting utilities (formatDocumentText) and
provide the critical transformation layer between repository scanning
and vector storage.

Added 29 comprehensive tests including:
- Document transformation with full metadata
- Single and batch operations
- Export status filtering (public API extraction)
- Type-based filtering (functions, classes, etc.)
- Case-insensitive language filtering
- Integration scenarios (chained filters + preparation)

100% coverage on all document utilities.

Wire up all utility modules and update RepositoryIndexer:

Integration changes:
- Create utils/index.ts barrel export for clean imports
- Update RepositoryIndexer to import from modular utils
- Remove old private methods (now extracted as utilities):
  * prepareDocumentsForEmbedding → utils/documents.ts
  * formatDocumentText → utils/formatting.ts
  * getExtensionForLanguage → utils/language.ts

Testing:
- All 126 tests passing (87 utils + 39 integration)
- Backward compatibility maintained
- No behavior changes, only refactoring

This completes the modular refactoring, providing:
- Clean separation of concerns (language → formatting → documents)
- Tree-shakeable exports for optimal bundling
- Self-contained, testable modules
- 100% coverage on all utility modules

Benefits:
- Improved testability (direct unit tests vs integration-only)
- Better code organization (SRP)
- Easier to understand and maintain
- Ready for reuse in other packages

Add comprehensive documentation and tooling to make testability
the default way of working:

Documentation:
- TESTABILITY.md: Complete guide with principles, examples, checklists
- FEATURE_TEMPLATE.md: Step-by-step template for new features
- Updated CONTRIBUTING.md with testability section
- PR template with testability checklist

Key principles enshrined:
1. Extract pure functions to utils/ modules
2. 100% coverage on utilities
3. No non-null assertions (!)
4. Organize by domain, not "misc"
5. Atomic commits with clear dependencies

Coverage targets:
- Pure utilities: 100%
- Integration: >80%
- CLI/UI: >60%

Scripts:
- Added test:coverage command for easy coverage checks

Real examples referenced:
- Explorer subagent (99 tests, 100% utils coverage)
- Repository indexer (87 tests, 100% utils coverage)

This establishes testability as a first-class concern and provides
clear guidance for all future development.