[FEATURE] Cypher-like graph query engine for MCP tool `query_graph`

[Wolfvin]

Problem

Agents currently must chain multiple MCP tools (trace -> impact -> context) to answer structural questions. Each hop costs tokens and round-trips. A single expressive query could replace 3-5 tool calls.

Proposed Change

Add a query_graph MCP tool that accepts an openCypher-subset query:

MATCH (f:Function)-[:CALLS]->(g) WHERE f.name = 'handleRequest' RETURN g.name, g.file
MATCH (c:Class)-[:INHERITS]->(p) WHERE p.name = 'BaseModel' RETURN c.name
MATCH (f:Function) WHERE NOT EXISTS { ()-[:CALLS]->(f) } RETURN f.name  -- dead code

Scope (minimal viable)

• MATCH, WHERE, RETURN, LIMIT
• Node labels + edge types
• Basic predicates: =, CONTAINS, IS NULL, NOT EXISTS
• Read-only (no write clauses)

Why This Matters for Agents

One query replaces: search -> trace -> filter -> format. Estimated 60-80% reduction in tool calls for structural questions. Agents can express precise intent instead of approximating with multiple tools.