feat: add entity graph report use case

[srikanth-11200]

Summary

Adds the Entity Graph Report use case for SuperDocs Task 2.

What it demonstrates

• Evidence-grounded report generation from related documents
• Entity-relationship graph and relationship diagram
• SuperDocs document upload and editing workflow
• Proposed-change approval workflow
• Document export
• Image upload
• Reproducible fixtures and demo

Included

• demo.py — end-to-end runnable demo
• report_to_superdocs.py — SuperDocs integration workflow
• superdocs_client.py — API client
• fixtures/approved_sections.json — approved evidence-grounded sections
• fixtures/diagram.svg — generated relationship diagram
• evidence/exported-report.docx — evidence from the live SuperDocs round trip
• screenshot.png — relationship graph screenshot
• README.md — setup, usage, evidence, and limitations

Testing

The complete upload → edit → approve → export workflow was tested successfully.

The DOCX image-export limitation is documented in the README. The relationship diagram is included separately as a fixture and screenshot.

Built for the SuperDocs Task 2 engineering assignment.