Add Java Enterprise Engineering Skills

[UsmanovMahmudkhan]

This PR adds a Java-focused enterprise engineering skill pack for AI coding agents and developers working with Java and Spring Boot.

It covers:

• Java SOLID principles
• Java Clean Code guidance
• Java design patterns
• safe refactoring
• TDD and test strategy
• Spring Boot Clean Architecture
• Spring Boot Hexagonal Architecture
• basic Domain-Driven Design
• Java/Spring security review
• common Java code smells
• enterprise Java review checklists
• reusable agent output formats
• agent review workflow routing, severity rules, and self-review gates
• review examples for converting weak findings into evidence-backed Java/Spring comments
• a dependency-free quality gate script for validating the skill pack

The goal is to extend the existing repository with practical Java/Spring guidance that is specific, reviewable, original, directly usable by coding agents, and easier to maintain over time.

Checklist:

• Added Java Enterprise Engineering skill
• Added Java/Spring-specific reference documents
• Added architecture, testing, security, refactoring, and review guidance
• Added strict agent output formats
• Added agent review workflow and review examples
• Added dependency-free quality gate script
• Updated README with the new skill and references
• Checked Markdown formatting
• Removed vague generic advice
• Avoided copied text/code
• Kept the PR focused and non-disruptive

Checks run:

• bash scripts/quality-gate.sh
• git diff --check