Issue 13 hybrid search

[zohaib-7035]

This PR implements the Hybrid Search for the Knowledge Space Agent, combining Vector Search and Keyword Search using Reciprocal Rank Fusion (RRF).

Changes:

• Added hybrid_search method in retrieval.py
• Added reciprocal_rank_fusion function
• Added placeholder keyword_search method for future keyword integration
• Updated .env.template if required

This addresses Issue part of #13.

[QuantumByte-01]

Closing this PR for a critical security issue: a real Google API key is committed in .env.template (AIzaSyBSW2NPqACOJnvm396j6nt3PydiEwJ2-HU). This key must be rotated immediately. Please revoke it in Google Cloud Console, never commit real credentials even in template files — use placeholder values like your_api_key_here instead. Beyond that, the RRF implementation is structurally sound but keyword_search is a placeholder that always returns [], making the hybrid search pure vector search. Please fix both issues and re-open a clean PR.