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/shared/constants/rules.ts

@@ -10,6 +10,7 @@ export const RULES = {
     ENTITY_EXPOSURE: "entity-exposure",
     DEPENDENCY_DIRECTION: "dependency-direction",
     REPOSITORY_PATTERN: "repository-pattern",
+    AGGREGATE_BOUNDARY: "aggregate-boundary",
 } as const
 
 /**