Use neo4j to store knowledge graph

[superstar54]

Knowledge graphs are persisted to Neo4j (configure via NODE_GRAPH_NEO4J_URI, NODE_GRAPH_NEO4J_USER, NODE_GRAPH_NEO4J_PASSWORD).

[codecov]

Codecov Report

❌ Patch coverage is 63.68330% with 211 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.73%. Comparing base (0154414) to head (f290f65).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_graph_engine/cli.py	35.22%	114 Missing ⚠️
src/node_graph_engine/neo4j/knowledge_graph.py	74.67%	77 Missing ⚠️
src/node_graph_engine/property.py	36.36%	7 Missing ⚠️
src/node_graph_engine/core/execution.py	62.50%	6 Missing ⚠️
src/node_graph_engine/core/utils.py	45.45%	6 Missing ⚠️
tests/test_cli_knowledge_graph.py	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
- Coverage   74.07%   72.73%   -1.35%     
==========================================
  Files          31       34       +3     
  Lines        2527     2927     +400     
==========================================
+ Hits         1872     2129     +257     
- Misses        655      798     +143     

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