refactor: Reorganize docs folder structure for clarity and version management (PR 3c)

Reorganize documentation following AGENTS.md v16.0 governance (traceability, reproducibility):

Structure changes:
- CURRENT/ (new)
  ├─ 00~08.md (v16.0 standards, renamed for clarity)
  └─ CATALOGS/ (9 CSV files: WBS, decision log, debt register, matrices, catalogs)

- LEGACY/ (new, read-only archives)
  ├─ v11/ (original baseline + hardening analysis)
  ├─ v12~v15/ (.gitkeep + README for future archiving)

- DECISIONS/ (new, ready for ADR usage)
- TEMPLATES/ (existing, unchanged)

Deletions (consolidated into CURRENT/):
- v16_0/ folder (files migrated)
- hardening/ folder (contents → LEGACY/v11/)
- Root-level v11 files (00~07.md, CSV)

Renames (for clarity):
- 00_EXECUTIVE_REFERENCE_IMPLEMENTATION.md → 00_EXECUTIVE.md
- 01_BRUTAL_ROLE_AUDIT.md → 01_ROLE_AUDIT.md
- 02_FRONTEND_ADAPTER_CRUD_STANDARD.md → 02_FE_ADAPTER.md
- 03_BACKEND_DATA_SCHEDULER_STANDARD.md → 03_BE_DATA.md
- 04_ALGORITHM_MODEL_GOVERNANCE.md → 04_ALGORITHM.md
- 05_PROCESS_VIBE_DEBT_CONTROL.md → 05_PROCESS_VIBE_DEBT.md
- 06_VALIDATION_TRUTH.md → 06_VALIDATION.md
- 07_PACKAGE_ATTACHMENT_POLICY.md → 07_PACKAGE_POLICY.md

Updates:
- docs/INDEX.md (complete rewrite with navigation)
- LEGACY/ folders with README + .gitkeep

Benefits:
✅ Clear version management (v16.0 is active, v11~v15 read-only)
✅ No version mixing in root
✅ CURRENT/ as single point of reference for active docs
✅ CATALOGS/ consolidates all data matrices
✅ LEGACY/ preserves history without clutter
✅ Traceability: decision log, tech debt, WBS all linked
✅ DECISIONS/ ready for ADR pattern (future use)

Sync with root:
- CLAUDE.md references: docs/CURRENT/, docs/INDEX.md ✅
- AGENTS.md references: docs/CURRENT/, traceability ✅
- README.md: Document guide links updated ✅

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/CURRENT/CATALOGS/SOURCE_COVERAGE_MATRIX.csv

@@ -0,0 +1,5 @@
+File,Relative_Path,Size,SHA256,Role,Package,Treatment
+K-ArtSell_퀀트투자자문_SI_본프로그램착수_누적고도화_통합실행기준서_v10.0(1).docx,attachments/current_session/K-ArtSell_퀀트투자자문_SI_본프로그램착수_누적고도화_통합실행기준서_v10.0(1).docx,402134,d86795186ec5f578741d64618eb53278a1c3a55795e73307e5bee245980a83a6,최상위 누적 실행기준서,CORE_AND_FULL,RETAINED_UNMODIFIED
+K-ArtSell_Aegis_v12_1_실행준비_통합고도화_제안서(1).docx,attachments/current_session/K-ArtSell_Aegis_v12_1_실행준비_통합고도화_제안서(1).docx,64063,eab0c2866b6c3d5d1f2fd8b9fbb33443e010e1adc2cfe612f27a98f97845e8b3,실행준비 통합 제안서,CORE_AND_FULL,RETAINED_UNMODIFIED
+K-ArtSell_Aegis_v15_0_통합고도화_제안서.docx,attachments/current_session/K-ArtSell_Aegis_v15_0_통합고도화_제안서.docx,52933,ef7e5746f129ecf14d1bc7d7bd5705fe44ca72251b050769506beb343e28c0a0,직전 통합 고도화 제안서,CORE_AND_FULL,RETAINED_UNMODIFIED
+KArtSell_Aegis_v15_0_Core_NoLegacy(1).zip,attachments/source_archives/KArtSell_Aegis_v15_0_Core_NoLegacy(1).zip,4390109,6c88d2442c831fa11d42726951592929caf2b5bd5847e6b42f9ad9ef28ee1b95,직전 Core 구현 기준선,FULL_ONLY,RETAINED_UNMODIFIED