FEAT: HiRAG Hierarchical RAG

[github-actions]

🎯 Objective

Implement the core functionality for the HiRAG Hierarchical RAG feature.

📦 Crate

$(@{id=hirag-implementation; name=HiRAG Hierarchical RAG; phase=2; crate=synapse-core; passes=False; tests=System.Object[]; last_updated=}.crate)

📚 Description from features.json

HiRAG Hierarchical RAG

This task was automatically generated by the Planner Agent based on the project's feature manifest. The goal is to implement this feature and ensure all related tests pass.