feat(http_server): add section headers to _payload_to_text for model parsing

The flattened text output now uses markdown headers so the model can
distinguish L0 identity from L1 summary from L2 episodes from handles.
This replaces undifferentiated newline-separated text with structured
sections: ## L0 Identity, ## L1 Recent Summary, ## Rich Club,
## L2 Episode, ## Handles.

src/iai_mcp/http_server.py

@@ -73,16 +73,17 @@ def _payload_to_text(result: Any) -> str:
     if not isinstance(result, dict):
         return str(result)
     parts: list[str] = []
+    headers = {"l0": "## L0 Identity", "l1": "## L1 Recent Summary", "rich_club": "## Rich Club"}
     for key in ("l0", "l1", "rich_club"):
         val = result.get(key)
         if isinstance(val, str) and val.strip():
-            parts.append(val.strip())
+            parts.append(f"{headers[key]}\n{val.strip()}")
     l2 = result.get("l2")
     if isinstance(l2, list):
         for item in l2:
             text = item if isinstance(item, str) else json.dumps(item, ensure_ascii=False)
             if text.strip():
-                parts.append(text.strip())
+                parts.append(f"## L2 Episode\n{text.strip()}")
     # Compact handles (populated at minimal; also present at standard/deep).
     handles = [
         result.get(k)
@@ -90,7 +91,7 @@ def _payload_to_text(result: Any) -> str:
     ]
     handles = [h for h in handles if isinstance(h, str) and h.strip()]
     if handles:
-        parts.append(" ".join(handles))
+        parts.append(f"## Handles\n{' '.join(handles)}")
     return "\n\n".join(parts)