refactor: consolidate LLM JSON parsing into shared llm_utils module

Extracted duplicate _strip_markdown_fences / _parse_llm_json functions
from 7 files into app/services/llm_utils.py. Two shared functions:
- strip_markdown_fences(): fence stripping only
- parse_llm_json(): fence stripping + JSON parse + error logging

Files updated: flowpilot_engine, knowledge_flywheel, session_to_flow_service,
ai_tree_generator_service, ai_fix_service, ai_chat_service, kb_conversion_service

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/knowledge_flywheel.py

@@ -20,6 +20,7 @@ from sqlalchemy.orm import selectinload
 
 from app.core.ai_provider import get_ai_provider
 from app.core.config import settings
+from app.services.llm_utils import parse_llm_json
 from app.services.notification_service import notify
 from app.models.ai_session import AISession
 from app.models.ai_session_step import AISessionStep
@@ -316,7 +317,7 @@ async def _propose_new_flow(session: AISession, db: AsyncSession) -> None:
             max_tokens=4096,
         )
 
-        parsed = _parse_llm_json(raw_response)
+        parsed = parse_llm_json(raw_response)
     except Exception as e:
         logger.warning("Knowledge Flywheel LLM call failed for session %s: %s", session.id, e)
         return
@@ -407,7 +408,7 @@ async def _propose_enhancement(session: AISession, db: AsyncSession) -> None:
             max_tokens=4096,
         )
 
-        parsed = _parse_llm_json(raw_response)
+        parsed = parse_llm_json(raw_response)
     except Exception as e:
         logger.warning("Knowledge Flywheel enhancement LLM call failed for session %s: %s", session.id, e)
         return
@@ -451,18 +452,3 @@ async def _propose_enhancement(session: AISession, db: AsyncSession) -> None:
     )
 
 
-def _parse_llm_json(raw_text: str) -> dict[str, Any]:
-    """Parse JSON from LLM response, handling common quirks."""
-    text = raw_text.strip()
-
-    # Strip markdown code fences if present
-    if text.startswith("```"):
-        lines = text.split("\n")
-        lines = [line for line in lines if not line.strip().startswith("```")]
-        text = "\n".join(lines).strip()
-
-    try:
-        return json.loads(text)
-    except json.JSONDecodeError as e:
-        logger.warning("Knowledge Flywheel JSON parse failed: %s — raw: %.300s", e, text)
-        raise ValueError(f"Invalid JSON from LLM: {e}") from e