fix(lead-stage): escape curly braces in STYLIST_SYSTEM_PROMPT before format

Prompt contains JSON examples with { } that confused .format().
Escape all braces first, then substitute only stage_injection.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/agent/lead_stage_agent/graph.py

@@ -555,7 +555,8 @@ class LeadStageGraph:
 
         injection = format_insight_injection(old_insight) if old_insight else "Chưa có insight — đây là lần đầu."
 
-        stylist_sys = STYLIST_SYSTEM_PROMPT.format(stage_injection=injection)
+        # Escape curly braces in prompt, then substitute only stage_injection
+        stylist_sys = STYLIST_SYSTEM_PROMPT.replace("{", "{{").replace("}", "}}").replace("{{stage_injection}}", "{stage_injection}").format(stage_injection=injection)
 
         # Build messages cho Stylist
         stylist_messages: list[BaseMessage] = [SystemMessage(content=stylist_sys)]