feat: expose Cypher export on he-mcp

[Frankie-Xu]

Summary

• MCP export_cypher and he export cypher call export_ka_cypher.
• The adapter uses require_graph_ka + resolve_export_file, then the existing export_to_cypher encoder (no rewrite).
• N-ary edges stay :Hyperedge + :IN (no pairwise clique).
• GRAPH_TYPE_ERROR now names Cypher alongside GraphML/CSV/JSON-LD.

Closes yifanfeng97#143

Out of scope

• Live Neo4j / neo4j or pyarrow packages
• CREATE NODE TABLE dialects / Ladybug / Arrow bulk load
• Encoder rewrite
• JSON-LD MCP (separate PR)

Test plan

• uv run pytest tests/test_mcp_server.py tests/utils/test_exporters.py tests/cli/ -q
• Adapter Cypher matches export_to_cypher; 3-ary script has :Hyperedge / [:IN] and no -[:REL] clique
• Existing nonempty file refused without overwrite
• Non-graph KA returns the shared GRAPH_TYPE_ERROR including Cypher

  

[Frankie-Xu]

Opened on the fork by mistake; tracked upstream as yifanfeng97#147. Closing.