Domain entity pages show no inter-domain relationships

[jonathanpopham]

Problem

Domain entity pages on repos.supermodeltools.com show no connections between domains. The Supermodel API returns inter-domain relationships (e.g., DOMAIN_RELATES, dependsOn, processes_requests_from) in graph.relationships, but graph2md silently drops them because the relationship switch block in internal/graph2md/graph2md.go has no handler for domain-to-domain relationship types.

Expected Behavior

When the Supermodel graph contains relationships between Domain nodes, the generated markdown should:

1. Include a "Related Domains" section in the domain body
2. Show related domains in the Mermaid diagram with labeled arrows
3. Include related domain edges in the graph_data JSON frontmatter

Root Cause

The relationship indexing switch block (~line 167) only handles known types (IMPORTS, calls, belongsTo, etc.). Any relationship type connecting two Domain nodes falls through silently. No domainEdge struct or index maps exist for domain-to-domain relationships.

Fix

• Add a domainEdge struct and domainRelatesOut/domainRelatesIn index maps
• Catch domain-to-domain relationships in a default case
• Thread the new maps through renderContext
• Update writeGraphData(), writeMermaidDiagram(), and writeDomainBody() for Domain entities

Migrated from https://github.com/supermodeltools/graph2md/issues/1