Implements neighbor, path, and subgraph queries over the fact graph.
Enables: "What depends on X?", "What is connected to Y?" queries.
- scripts/graph_query.py: CLI tool with neighbors/path/subgraph/stats
- scripts/test_graph_query.py: comprehensive unit + CLI tests
- Handles 10K nodes in <20ms (requirement: <1s)
- Outputs JSON for machine consumption
Closes#150
