docs: Enhance document routing and create tech debt registry (PR 3a)

Strengthen documentation coherence and navigation:

README.md:
- Add 'Developer's Document Guide' section at top
- Link to CLAUDE.md, AGENTS.md, architecture, guardrails, validation gates
- Reference docs/INDEX.md and TECH_DEBT_REGISTER.md

docs/INDEX.md (new):
- Central documentation index with purpose and audience
- Core guides, executive/planning, tech debt, reference docs
- Quick links for first change, architectural decisions, validation

TECH_DEBT_REGISTER.md (new):
- Tech debt tracking per AGENTS.md v16.0
- 8 registered debts (code analysis suppressions, deferred refactoring)
- Impact/Effort matrix with paydown strategy (quarterly 20% target)
- Status tracking with completion history
- How-to guide for resolving debt

CLAUDE.md:
- Update TECH_DEBT_REGISTER reference from 'to be created' to actual link

Result:
- Developers land on README → directed to CLAUDE.md or AGENTS.md
- All docs cross-reference each other for circular navigation
- Tech debt fully visible and tracked with clear paydown roadmap
- docs/INDEX.md provides comprehensive reference point

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

README.md

@@ -2,10 +2,26 @@
 
 Reference implementation closure for governed advisory research. `RESEARCH_CANDIDATE_NOT_PRODUCTION`; automatic order/KIS/model activation OFF.
 
-
 **상태:** `IMPLEMENTATION_TEMPLATE / STATIC_VALIDATED / BUILD_DB_E2E_SHADOW_REHEARSAL_REQUIRED`  
 **운영 경계:** `RESEARCH_CANDIDATE_NOT_PRODUCTION · 투자자문형 우선 · 자동주문/KIS 제출 OFF · 자동 모델승격 OFF`
 
+---
+
+## 📖 개발자를 위한 문서 가이드
+
+**→ 여기서 시작:**
+- **Claude Code 개발:** [`CLAUDE.md`](CLAUDE.md) — 로컬 개발, 빌드, 테스트, Gitea API
+- **AI 코딩 헌법:** [`AGENTS.md`](AGENTS.md) — 모든 작업의 기준 (13개 결정 기준, 체크리스트, 안티패턴)
+- **아키텍처:** [`docs/03_ARCHITECTURE_BE_FE.md`](docs/03_ARCHITECTURE_BE_FE.md) — Modular Monolith, Vertical Slice, Dapper, Hangfire
+- **바이브 코딩 가이드:** [`docs/06_VIBE_CODING_GUARDRAILS.md`](docs/06_VIBE_CODING_GUARDRAILS.md) — AI 입력 패킷, 차단 규칙, 리팩터링 정공법
+- **Validation Gates:** [`docs/07_VALIDATION_GATES.md`](docs/07_VALIDATION_GATES.md) — 생산 배포 조건
+
+**문서 색인:** [`docs/INDEX.md`](docs/INDEX.md)
+
+**기술부채 추적:** [`TECH_DEBT_REGISTER.md`](TECH_DEBT_REGISTER.md) — Impact/Effort 매트릭스, 분기별 회수 목표
+
+---
+
 v16.0은 v10.0의 Baseline Preservation + Delta + Supersession + Traceability, RG-01~08, PIT·불변 Evidence·26 Vertical Slice 기준과 v12.1~v14.0 실행 패키지를 삭제하지 않고 보존한 누적 Delta다. 화면 수나 코드량을 생산준비도로 간주하지 않으며, `Source/License → PIT/Revision/DQ → Evidence/Version Set → Sell/Reentry → Recommendation → Maker-Checker → Publish → Outcome → Reconciliation/DR`의 완결성으로 판단한다.
 
 ## v16.0 핵심 Delta