docs: add Phase Z implementation issue backlog

- Add docs/architecture/PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md
  as implementation issue inventory (28 issues + frontend pending)
  bridged from PHASE-Q-INSIGHT-TO-22STEP-MAP.md
- Sections: §1 22-step map (11) / §2 separate axis registry (9)
  / §3 audit §1 mapping correction (5) / §4 cleanup (3) /
  §5 frontend matching source pending
- Includes review loop workflow (Codex+Claude review thread) +
  8 labels + dependency convention (none/hard link/soft link)
- Register the new doc in PHASE-Z-ROADMAP.md §8

docs/architecture/PHASE-Z-ROADMAP.md

@@ -278,6 +278,7 @@ frontend (`design_agent_front/design-agent`) 와 backend phase_z2 의 연결 axi
 | **`PHASE-Z-ROADMAP.md`** (본 문서) | **앞으로의 진행 순서 / 큰 axis / 우선순위** | **자주** |
 | [`PHASE-Q-AUDIT.md`](PHASE-Q-AUDIT.md) | Phase Q 모듈의 Keep / Migrate / Reference / Delete 결정 + Salvage Plan + ROADMAP §7-B 우선순위 재정렬의 input | audit 진행 중 (모듈별로 1 turn) |
 | [`PHASE-Q-INSIGHT-TO-22STEP-MAP.md`](PHASE-Q-INSIGHT-TO-22STEP-MAP.md) | audit insight + frontend matching insight(source pending) → 22-step pipeline 매핑 + implementation 전 prereq lock | implementation axis 활성 시 |
+| [`PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md`](PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md) | implementation 단위 issue list — 22-step map / 별 axis registry / mapping correction / cleanup / frontend pending 통합 backlog | issue cycle 활성 시 |
 | `PROCESS_OVERVIEW.html` | 보고 / 피드백용 시각 프로토타입 | 보고 시점 |
 
 각 문서가 자기 역할만 하면 drift / 중복 / outdated 방지.