DOCS. (cleanup) Archive obsolete documentation and update INDEX

Cleanup:
- Archived 20 obsolete PRD and review files to docs/_archive_obsolete/
- Updated docs/INDEX.md with new structure
- Added refactoring section reference
- Removed historical migration signoff

Impact:
- Documentation bloat reduced: 82 → 62 files (24% reduction)
- Clearer organization with active vs archived docs
- INDEX.md now references current analysis documents

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

Author: xronocode

.DS_Store

@@ -11,6 +11,7 @@
 **/DerivedData/
 Icon?
 **/Pods/
+**/build/
 **/.symlinks/
 profile
 xcuserdata

apps/.DS_Store

@@ -1,8 +1,8 @@
 # MathMagic Documentation Index
 
-> **Last Updated**: 2025-06-17  
-> **Version**: 1.0  
-> **Status**: Production Ready
+> **Last Updated**: 2026-02-18
+> **Version**: 2.0
+> **Status**: Active Development — Phase 6 (Riverpod Migration)
 
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@@ -13,6 +13,8 @@
 | **Project Backbone** | [overview/project-backbone.md](overview/project-backbone.md) | Central hub for project information |
 | **Getting Started** | [overview/project-backbone.md#getting-started](overview/project-backbone.md) | Setup and onboarding guide |
 | **Technical Debt** | [todos-debts/sprint-planning.md](todos-debts/sprint-planning.md) | Current debt status and planning |
+| **Frontend-Backend Gaps** | [refactoring/2026-02-15_frontend-backend-gaps.md](refactoring/2026-02-15_frontend-backend-gaps.md) | Integration audit |
+| **AGENTS.md** | [../AGENTS.md](../AGENTS.md) | AI agent instructions |
 
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@@ -26,7 +28,9 @@ docs/
 ├── todos-debts/        # Technical debt tracking
 ├── standards/          # Development standards
 ├── operations/         # Operations and runbooks
-└── prd/               # Product requirements (archived)
+├── refactoring/        # Code reviews and gap analysis
+├── prd/               # Active product requirements
+└── _archive_obsolete/ # Archived/obsolete documents
 ```
 
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