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/design_director.py

@@ -5,9 +5,7 @@ Step B: 프리셋 안에서 블록 매핑 + 글자 수 가이드 (Sonnet)
 """
 from __future__ import annotations
 
-import json
 import logging
-import re
 from pathlib import Path
 from typing import Any
 
@@ -15,6 +13,7 @@ import httpx
 import yaml
 
 from src.config import settings
+from src.json_utils import parse_json as _parse_json
 from src.sse_utils import stream_sse_tokens
 
 logger = logging.getLogger(__name__)
@@ -1066,38 +1065,3 @@ def _validate_height_budget(blocks: list[dict], preset: dict) -> list[dict]:
         })
 
     return overflows
-
-
-def _parse_json(text: str) -> dict[str, Any] | None:
-    """텍스트에서 JSON을 추출한다.
-
-    Kei API가 마크다운 리스트 접두사(- )를 붙여 응답하는 경우에도 처리.
-    """
-    # 전처리: 각 줄 앞의 마크다운 리스트 접두사(- ) 제거
-    lines = text.split("\n")
-    cleaned_lines = []
-    for line in lines:
-        stripped = line.lstrip()
-        if stripped.startswith("- "):
-            cleaned_lines.append(stripped[2:])
-        elif stripped.startswith("* "):
-            cleaned_lines.append(stripped[2:])
-        else:
-            cleaned_lines.append(stripped)
-    cleaned = "\n".join(cleaned_lines)
-
-    # 원본 먼저 시도 → 클린 버전 시도
-    for target in [text, cleaned]:
-        patterns = [
-            r"```json\s*(.*?)```",
-            r"```\s*(.*?)```",
-            r"(\{.*\})",
-        ]
-        for pattern in patterns:
-            match = re.search(pattern, target, re.DOTALL)
-            if match:
-                try:
-                    return json.loads(match.group(1).strip())
-                except json.JSONDecodeError:
-                    continue
-    return None