refactor(#28): IMP-28 L4 _parse_json dedup (4 modules -> src/json_utils)

Consolidate duplicate _parse_json helpers from content_editor.py /
design_director.py / kei_client.py (fuller form) and pipeline.py (simple form)
into shared src/json_utils.parse_json (strict superset). All 18 call-sites
preserved via `parse_json as _parse_json` alias import; no behavior change.

- src/json_utils.py (new): shared helper, fenced/plain-fence/bare-brace patterns
  + list-prefix cleanup fallback.
- tests/test_json_utils.py (new): 9 unit tests pinning parser semantics.
- src/content_editor.py / design_director.py: remove local helper +
  unused `import json` / `import re`.
- src/kei_client.py / pipeline.py: remove local helper; `json` / `re` retained
  (used elsewhere).

Targeted tests 9 passed; full pytest 374 passed (3 pre-existing scripts/
collection errors reproduce on baseline 909bf75, IMP-28 unrelated).

src/json_utils.py

@@ -0,0 +1,46 @@
+"""JSON 추출 공용 유틸리티.
+
+Kei / Claude API 응답 텍스트에서 JSON 객체를 추출한다.
+content_editor, design_director, kei_client, pipeline 공통 헬퍼.
+
+응답이 마크다운 리스트 접두사("- " / "* ")로 감싸진 경우에도 처리.
+"""
+from __future__ import annotations
+
+import json
+import re
+from typing import Any
+
+_JSON_PATTERNS: tuple[str, ...] = (
+    r"```json\s*(.*?)```",
+    r"```\s*(.*?)```",
+    r"(\{.*\})",
+)
+
+
+def parse_json(text: str) -> dict[str, Any] | None:
+    """텍스트에서 JSON을 추출한다.
+
+    Kei API가 마크다운 리스트 접두사(- )를 붙여 응답하는 경우에도 처리.
+    원본 → 리스트 접두사 제거 버전 순서로 fenced JSON / plain fenced / 베어 brace 패턴을
+    차례로 시도한다. 모두 실패하면 None.
+    """
+    lines = text.split("\n")
+    cleaned_lines: list[str] = []
+    for line in lines:
+        stripped = line.lstrip()
+        if stripped.startswith("- ") or stripped.startswith("* "):
+            cleaned_lines.append(stripped[2:])
+        else:
+            cleaned_lines.append(stripped)
+    cleaned = "\n".join(cleaned_lines)
+
+    for target in (text, cleaned):
+        for pattern in _JSON_PATTERNS:
+            match = re.search(pattern, target, re.DOTALL)
+            if match:
+                try:
+                    return json.loads(match.group(1).strip())
+                except json.JSONDecodeError:
+                    continue
+    return None