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

skills/efcore-d2-db-diagram/references/efcore-model-extraction.md

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+# EF Core Model Extraction
+
+## Files to inspect
+
+Inspect, in this order:
+
+1. `DbContext` classes.
+2. `DbSet<T>` declarations.
+3. `OnModelCreating`.
+4. `IEntityTypeConfiguration<T>` classes.
+5. Entity classes.
+6. Migrations and model snapshot.
+7. Data annotations.
+
+## Mapping priority
+
+When sources conflict, use:
+
+1. Latest migration / model snapshot.
+2. Fluent API.
+3. Data annotations.
+4. EF Core conventions.
+5. C# shape.
+
+## Important EF Core APIs
+
+Look for:
+
+- `ToTable`
+- `HasKey`
+- `HasAlternateKey`
+- `HasIndex`
+- `IsUnique`
+- `Property`
+- `HasColumnName`
+- `HasColumnType`
+- `IsRequired`
+- `HasMaxLength`
+- `HasConversion`
+- `HasOne`
+- `WithMany`
+- `WithOne`
+- `HasForeignKey`
+- `OnDelete`
+- `OwnsOne`
+- `OwnsMany`
+- `UsingEntity`
+- `Ignore`
+
+## Migrations
+
+Use migrations to detect:
+
+- Actual table names.
+- Join tables.
+- Shadow FK columns.
+- Indexes.
+- Composite keys.
+- Delete behaviors.
+- Migration-only tables.