enhancement: Make result fusion weights configurable via env vars

[QuantumByte-01]

Problem

In fuse_results (agents.py), vector and KS search results are merged with hardcoded weights:

"final_score": res.get("similarity", 0) * 0.6   # vector
"final_score": res.get("_score", 0) * 0.4        # KS

No explanation for the 0.6/0.4 split. These cannot be tuned without code changes.

Fix

Make them env-var configurable:

VECTOR_WEIGHT = float(os.getenv("FUSION_VECTOR_WEIGHT", "0.6"))
KS_WEIGHT = float(os.getenv("FUSION_KS_WEIGHT", "0.4"))

Add to .env.template with a comment explaining the tradeoff.