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/core/ai_fix_service.py

@@ -86,11 +86,7 @@ def _serialize_tree_outline(
     return "\n".join(lines)
 
 
-def _strip_markdown_fences(text: str) -> str:
-    """Strip ```json...``` fences from AI response."""
-    return re.sub(r"^```(?:json)?\s*\n?", "", text.strip(), flags=re.MULTILINE).rstrip(
-        "`"
-    ).strip()
+from app.services.llm_utils import strip_markdown_fences as _strip_markdown_fences
 
 
 def _replace_node_in_tree(