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

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.