docs: Add comprehensive analysis of issue #3126 and PR #3113 review

This analysis reviews the callback system refactoring proposal, identifying:
- Code duplication claims (validated as accurate)
- Proposed solution design (well-implemented with minor concerns)
- Breaking change risk (removal of canonical_*_callbacks properties)
- Unused code (CallbackExecutor not utilized due to signature mismatches)
- Testing gaps (no integration test verification)

Recommendation: Request changes before merging
- Must fix: Add deprecation warnings instead of removing properties
- Must fix: Verify existing callback tests pass
- Should fix: Remove or document unused CallbackExecutor class

Overall: Excellent first contribution with high-quality implementation,
pending resolution of API compatibility and testing verification.

Author: Jacksunwei

analysis/README.md

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+# Code Analysis Materials
+
+This directory contains detailed analyses and reviews of issues, pull requests, and code quality improvements for the ADK Python project.
+
+## Contents
+
+- **[issue-3126-pr-3113-review.md](./issue-3126-pr-3113-review.md)** - Comprehensive review of callback system refactoring
+  - Issue: #3126 - Code duplication in callback system
+  - PR: #3113 - Refactor callback system to eliminate code duplication
+  - Status: Pending maintainer review
+  - Recommendation: Request changes (deprecation + test verification needed)
+
+## Review Process
+
+Each analysis follows this structure:
+
+1. **Root Cause Analysis** - Understanding the problem
+2. **Proposed Solution Review** - Evaluating the implementation
+3. **Impact Analysis** - Breaking changes, performance, maintainability
+4. **Recommendations** - Actionable next steps for contributors and maintainers
+5. **Conclusion** - Summary and final verdict
+
+## Contributing
+
+When adding new analyses:
+
+1. Create a new branch: `analysis/issue-XXXX-description`
+2. Add markdown file: `issue-XXXX-analysis.md`
+3. Update this README
+4. Commit and push for future reference