feat(mux-loop): redesign role CLI flags with --creator and --planner, drop --target-agent

- Replace '--target-agent' with mandatory '--creator <session>' flag
- Add explicit rejection error for legacy '--target-agent'
- Add '--planner <session>' flag with fail-fast check when '--plan' is missing
- Implement 2-branch session validation ('is not registered' vs 'is not running')
- Update in-repo references in hooks, SKILL.md, INSTALL.md, and tests
- Add tests/test_loop_cli.py with 9 unit/contract tests

deploy/INSTALL.md

@@ -108,7 +108,7 @@ $ bash .agents/skills/multi-agent-mux-loop/scripts/run_loop.sh \
     --plan-talk 1 \
     --reviewer "reviewer-a,reviewer-b" \
     --max-loop 3 \
-    --target-agent my-project-dev-claude \
+    --creator my-project-dev-claude \
     --task "구현할 명확한 개발 작업 목표"
 ```
 *   `--max-loop`는 코드 오류 발견 시 최대 교정(반복 수정) 횟수 제한 가드레일 역할을 합니다.