chore(config): sync env example and context for ±3 defaults

Why:
- d5ccec2 locked ±3 in code; env template and domain glossary were still at 0/0.

Changes:
- NEIGHBOR_PREV/NEXT=3 in .env.example
- CONTEXT.md Neighbor Expansion defaults updated to 3/3

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -14,9 +14,9 @@ OLLAMA_BASE_URL=http://192.168.10.10:11435
 
 # Retrieval defaults
 TOP_K=5
-# Neighbor Expansion for fixed_size only (ADR-0023); 0 = off
-NEIGHBOR_PREV=0
-NEIGHBOR_NEXT=0
+# Neighbor Expansion for fixed_size only (ADR-0023); 3/3 = stabilized default after benchmark
+NEIGHBOR_PREV=3
+NEIGHBOR_NEXT=3
 
 # LLM generation parameters
 TEMPERATURE=0.0