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

skills/efcore-d2-db-diagram/references/d2-erd-style.md

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+# D2 ERD Style
+
+## Recommended header
+
+```d2
+vars: {
+  d2-config: {
+    layout-engine: elk
+    theme-id: 300
+  }
+}
+```
+
+## Table node
+
+```d2
+Clients: {
+  shape: sql_table
+  Id: uuid {constraint: primary_key}
+  Name: varchar(200)
+  Status: enum
+}
+```
+
+## Relationship
+
+```d2
+Offers.ClientId -> Clients.Id: "N:1"
+```
+
+## Styles
+
+```d2
+classes: {
+  join_table: {
+    style.stroke-dash: 4
+  }
+  technical: {
+    style.opacity: 0.55
+  }
+  optional_relation: {
+    style.stroke-dash: 3
+  }
+}
+```

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.

skills/efcore-d2-db-diagram/references/grouping-modes.md

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+# Grouping Modes
+
+## bounded-context
+
+Group tables by domain area using folder, namespace and naming clues.
+
+Examples:
+
+- Clients
+- Offers
+- Freelances
+- Billing
+- Audit
+- Identity
+
+## schema
+
+Group by database schema from `ToTable` or migrations.
+
+## namespace
+
+Group by C# namespace.
+
+## flat
+
+Do not create containers.
+
+Use flat mode for small schemas or when the user wants maximum compatibility.

skills/efcore-d2-db-diagram/references/quality-gate.md

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+# Quality Gate
+
+Before delivery:
+
+- Confirm the selected DbContext.
+- Confirm source files inspected.
+- Validate table names against Fluent API and migrations.
+- Include primary keys.
+- Include foreign keys.
+- Include cardinalities.
+- Include join tables unless hidden by user choice.
+- Include owned types according to user choice.
+- Hide technical tables only if configured and list them in the summary.
+- Run `d2 fmt` when available.
+- Use full dot-notation for edges inside containers.
+- Provide render command.

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

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+# 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.