chore: Update system design overview prompt

Author: GoodbyePlanet

server/prompts/system.py

@@ -55,6 +55,20 @@ def system_design_overview() -> str:
             "across the system.\n"
             "- **Key Architectural Patterns** — flag patterns you have evidence for: "
             "event-driven, CQRS, saga, API gateway, BFF, strangler fig, etc.\n"
+            "- **Diagrams** — once the investigation is complete, render the findings as "
+            "Mermaid diagrams in fenced ` ```mermaid ` code blocks:\n"
+            "  - A **system context / component diagram** (`flowchart LR` or `graph LR`) "
+            "showing every service, edge entry point, and data store as nodes; label "
+            "edges with the protocol (REST, gRPC, async) and direction.\n"
+            "  - A **data architecture diagram** (`flowchart` or `erDiagram`) showing "
+            "which service owns which store and any sharing.\n"
+            "  - If async messaging exists, add a **sequence diagram** "
+            "(`sequenceDiagram`) for one representative end-to-end flow (e.g. a request "
+            "that crosses multiple services or a key event chain).\n"
+            "  Only include diagrams for which you found concrete evidence in the code. "
+            "Keep node labels short and consistent with the names used in the Service "
+            "Inventory table.\n"
             "\n"
-            "Skip any section you have no evidence for. Do not invent details."
+            "Skip any section (including diagrams) you have no evidence for. "
+            "Do not invent details."
         )