refactor(client): extract disk-writer.ts into focused modules

Split 1,478-line file into 4 focused sub-modules:
- nodejs-modules.ts: Dynamic imports and initialization
- file-operations.ts: Atomic writes, locking, directory management
- entity-extraction.ts: Entity info extraction and path generation
- index-manager.ts: Hierarchical index.json operations

Benefits: Single Responsibility Principle, improved testability,
better maintainability. Main orchestrator now 377 lines vs 1,478.
Re-exports types for backward compatibility. No breaking changes.

Author: Mearman