Add Cypher query language support

[villelaitila]

Summary

• Add openCypher query support for sgraph models via a pluggable sPyCy backend (src/sgraph/cypher.py)
• Document graph conventions for software architecture and genealogy domains (docs/graph-conventions.md)
• Full documentation with examples, CLI reference, and feature matrix (docs/cypher.md)

What it does

Enables querying sgraph models with Cypher pattern matching:

-- Find transitive dependencies
MATCH (a:file)-[:imports|function_ref*1..3]->(b)
RETURN DISTINCT a.name, b.name

-- Trace ancestors in a genealogy model
MATCH (p)-[:parent*1..3]->(ancestor)
WHERE p.name CONTAINS 'Pekka'
RETURN ancestor.name

Python API:

from sgraph.cypher import cypher_query
results = cypher_query(model, 'MATCH (n:file) RETURN n.name')

CLI with interactive REPL:

python -m sgraph.cypher model.xml.zip                          # REPL
python -m sgraph.cypher model.xml.zip 'MATCH ...' -f csv       # single query
python -m sgraph.cypher model.xml.zip -f dot 'MATCH ... RETURN a, r, b' | dot -Tpng -o out.png

11 output formats: table, csv, tsv, json, jsonl, xml, deps, dot, plantuml, graphml, cytoscape. Graph formats extract a subgraph from query results and export via existing sgraph converters.

Design

The SGraphCypherBackend maps sgraph's dual-layer model (element tree + association edges) to Cypher's labeled property graph:

sgraph	Cypher
Element type attr	Node label (:file, :class)
Association deptype	Relationship type (:imports, :inc)
Parent-child tree	:CONTAINS relationships (optional)
Element attrs + name/path	Node properties

Optional dependency: pip install spycy-aneeshdurg

Test plan

• Tabular queries: MATCH, WHERE, RETURN, ORDER BY, LIMIT, aggregations
• Variable-length paths (*1..N), OPTIONAL MATCH, WITH, UNION
• Label and property filtering
• All 5 tabular output formats (table, csv, tsv, json, jsonl)
• All 6 graph output formats (xml, deps, dot, plantuml, graphml, cytoscape)
• Subgraph extraction preserves element types, attributes, and associations
• CLI error handling (bad query, missing file)
• Interactive REPL with \format switching
• Both software architecture and genealogy conventions