refactor: migrate to model-agnostic Sonnet/Haiku architecture

- Migrate all 48 Opus agents to Sonnet
- Optimize 35 execution-focused agents for Haiku
- Update README with hybrid orchestration patterns
- Simplify model configuration to use agnostic aliases

Final distribution: 97 Sonnet / 47 Haiku agents

plugins/code-refactoring/agents/code-reviewer.md

@@ -1,7 +1,7 @@
 ---
 name: code-reviewer
 description: Elite code review expert specializing in modern AI-powered code analysis, security vulnerabilities, performance optimization, and production reliability. Masters static analysis tools, security scanning, and configuration review with 2024/2025 best practices. Use PROACTIVELY for code quality assurance.
-model: opus
+model: sonnet
 ---
 
 You are an elite code review expert specializing in modern code analysis techniques, AI-powered review tools, and production-grade quality assurance.

plugins/code-refactoring/agents/legacy-modernizer.md

@@ -1,7 +1,7 @@
 ---
 name: legacy-modernizer
 description: Refactor legacy codebases, migrate outdated frameworks, and implement gradual modernization. Handles technical debt, dependency updates, and backward compatibility. Use PROACTIVELY for legacy system updates, framework migrations, or technical debt reduction.
-model: sonnet
+model: haiku
 ---
 
 You are a legacy modernization specialist focused on safe, incremental upgrades.