Add knowledge_graph attr to Graph

[superstar54]

Move the knowledge graph-related code from the node-graph-engine repo to here because the knowledge graph itself is agnostic to engines.

[codecov]

Codecov Report

❌ Patch coverage is 93.00000% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.65%. Comparing base (4d15dcc) to head (8fc0ad4).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_graph/knowledge_graph.py	90.25%	39 Missing ⚠️
src/node_graph/semantics.py	79.41%	7 Missing ⚠️
tests/test_knowledge_graph.py	98.78%	2 Missing ⚠️
src/node_graph/utils/function.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #133      +/-   ##
==========================================
+ Coverage   90.54%   90.65%   +0.11%     
==========================================
  Files          73       74       +1     
  Lines        7125     7713     +588     
==========================================
+ Hits         6451     6992     +541     
- Misses        674      721      +47     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.