docs: add static factory method implementation plan

[JunggiKim]

Summary

• 71개 Predicate 클래스에 정적 팩토리 메서드 (instance()/of()) 추가를 위한 상세 구현 계획 문서
• 다른 컴퓨터에서 바로 구현 진행 가능하도록 모든 파일 경로, 코드 패턴, 테스트 데이터, 검증 단계를 포함
• 기존 API 하위 호환성 100% 유지

변경 내용

• plans.md 파일 추가 (760줄, 10개 Phase의 상세 구현 계획)

Test plan

• 문서만 추가하므로 빌드 영향 없음

🤖 Generated with Claude Code

[gitguardian]

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GitGuardian id	GitGuardian status	Secret	Commit	Filename
28257257	Triggered	JSON Web Token	cd94a4d	java-refined-kotlin/src/test/kotlin/io/github/junggikim/refined/kotlin/ScalarKotlinExtensionsTest.kt	View secret

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