feat: expose JSON-LD export on he-mcp

[Frankie-Xu]

Summary

• MCP export_jsonld and he export jsonld call export_ka_jsonld.
• The adapter uses require_graph_ka + resolve_export_file, then the existing export_to_jsonld encoder (no rewrite).
• Pairwise edges keep source/target; N-ary edges stay @type: Hyperedge.

Closes yifanfeng97#142

Out of scope

• schema.org / RDFLib
• Cypher MCP / GRAPH_TYPE_ERROR Cypher wording
• source_ids / write tools
• Changing GraphML/CSV encoding

Test plan

• uv run pytest tests/test_mcp_server.py tests/utils/test_exporters.py tests/cli/ -q
• Adapter JSON-LD matches export_to_jsonld (pairwise source/target, 3-ary Hyperedge)
• Existing nonempty file refused without overwrite; overwrite=true writes
• Non-graph KA returns the shared GRAPH_TYPE_ERROR

  

[Frankie-Xu]

Opened on the fork by mistake; tracked upstream as yifanfeng97#146. Closing.