feat: add efcore-d2-db-diagram skill and related documentation (#1821)

skills/efcore-d2-db-diagram/references/relationship-rules.md

@@ -0,0 +1,59 @@
+# Relationship Rules
+
+## One-to-many
+
+Detected from:
+
+- `HasOne(...).WithMany(...)`
+- FK property on dependent entity.
+- Collection navigation on principal entity.
+
+Render dependent to principal:
+
+```d2
+Orders.ClientId -> Clients.Id: "N:1"
+```
+
+## One-to-one
+
+Detected from:
+
+- `HasOne(...).WithOne(...)`
+- Unique FK index.
+- Shared primary key relationship.
+
+Render dependent to principal:
+
+```d2
+ClientProfiles.ClientId -> Clients.Id: "1:1"
+```
+
+## Many-to-many
+
+Detected from:
+
+- `UsingEntity`
+- Two collection navigations without explicit join entity.
+- Migration-created join table with two FKs and composite key.
+
+Render the join table explicitly by default.
+
+## Owned types
+
+Detected from:
+
+- `OwnsOne`
+- `OwnsMany`
+- `[Owned]`
+
+Inline by default unless table splitting or separate table mapping is detected.
+
+## Optional relationships
+
+A relationship is optional when:
+
+- FK is nullable.
+- `IsRequired(false)` is configured.
+- Migration column is nullable.
+
+Use dashed line for optional relationships.