feat: add aggregate boundary validation (v0.7.0)

Implement DDD aggregate boundary validation to detect and prevent direct
entity references across aggregate boundaries.

Features:
- Detect direct entity imports between aggregates
- Allow only ID or Value Object references
- Support multiple folder structures (domain/aggregates/*, domain/*, domain/entities/*)
- Filter allowed imports (value-objects, events, repositories, services)
- Critical severity level for violations
- 41 comprehensive tests with 92.55% coverage
- CLI output with detailed suggestions
- Examples of good and bad patterns

Breaking changes: None
Backwards compatible: Yes

packages/guardian/src/domain/value-objects/AggregateBoundaryViolation.ts

@@ -0,0 +1,137 @@
+import { ValueObject } from "./ValueObject"
+import { AGGREGATE_VIOLATION_MESSAGES } from "../constants/Messages"
+
+interface AggregateBoundaryViolationProps {
+    readonly fromAggregate: string
+    readonly toAggregate: string
+    readonly entityName: string
+    readonly importPath: string
+    readonly filePath: string
+    readonly line?: number
+}
+
+/**
+ * Represents an aggregate boundary violation in the codebase
+ *
+ * Aggregate boundary violations occur when an entity from one aggregate
+ * directly references an entity from another aggregate, violating DDD principles:
+ * - Aggregates should reference each other only by ID or Value Objects
+ * - Direct entity references create tight coupling between aggregates
+ * - Changes to one aggregate should not require changes to another
+ *
+ * @example
+ * ```typescript
+ * // Bad: Direct entity reference across aggregates
+ * const violation = AggregateBoundaryViolation.create(
+ *     'order',
+ *     'user',
+ *     'User',
+ *     '../user/User',
+ *     'src/domain/aggregates/order/Order.ts',
+ *     5
+ * )
+ *
+ * console.log(violation.getMessage())
+ * // "Order aggregate should not directly reference User entity from User aggregate"
+ * ```
+ */
+export class AggregateBoundaryViolation extends ValueObject<AggregateBoundaryViolationProps> {
+    private constructor(props: AggregateBoundaryViolationProps) {
+        super(props)
+    }
+
+    public static create(
+        fromAggregate: string,
+        toAggregate: string,
+        entityName: string,
+        importPath: string,
+        filePath: string,
+        line?: number,
+    ): AggregateBoundaryViolation {
+        return new AggregateBoundaryViolation({
+            fromAggregate,
+            toAggregate,
+            entityName,
+            importPath,
+            filePath,
+            line,
+        })
+    }
+
+    public get fromAggregate(): string {
+        return this.props.fromAggregate
+    }
+
+    public get toAggregate(): string {
+        return this.props.toAggregate
+    }
+
+    public get entityName(): string {
+        return this.props.entityName
+    }
+
+    public get importPath(): string {
+        return this.props.importPath
+    }
+
+    public get filePath(): string {
+        return this.props.filePath
+    }
+
+    public get line(): number | undefined {
+        return this.props.line
+    }
+
+    public getMessage(): string {
+        return `${this.capitalizeFirst(this.props.fromAggregate)} aggregate should not directly reference ${this.props.entityName} entity from ${this.capitalizeFirst(this.props.toAggregate)} aggregate`
+    }
+
+    public getSuggestion(): string {
+        const suggestions: string[] = [
+            AGGREGATE_VIOLATION_MESSAGES.USE_ID_REFERENCE,
+            AGGREGATE_VIOLATION_MESSAGES.USE_VALUE_OBJECT,
+            AGGREGATE_VIOLATION_MESSAGES.AVOID_DIRECT_REFERENCE,
+            AGGREGATE_VIOLATION_MESSAGES.MAINTAIN_INDEPENDENCE,
+        ]
+
+        return suggestions.join("\n")
+    }
+
+    public getExampleFix(): string {
+        return `
+// ❌ Bad: Direct entity reference across aggregates
+// domain/aggregates/order/Order.ts
+import { User } from '../user/User'
+
+class Order {
+    constructor(private user: User) {}
+}
+
+// ✅ Good: Reference by ID
+// domain/aggregates/order/Order.ts
+import { UserId } from '../user/value-objects/UserId'
+
+class Order {
+    constructor(private userId: UserId) {}
+}
+
+// ✅ Good: Use Value Object for needed data
+// domain/aggregates/order/value-objects/CustomerInfo.ts
+class CustomerInfo {
+    constructor(
+        readonly customerId: string,
+        readonly customerName: string,
+        readonly customerEmail: string
+    ) {}
+}
+
+// domain/aggregates/order/Order.ts
+class Order {
+    constructor(private customerInfo: CustomerInfo) {}
+}`
+    }
+
+    private capitalizeFirst(str: string): string {
+        return str.charAt(0).toUpperCase() + str.slice(1)
+    }
+}