fix(mcp): /reload-mcp now updates agent tools + injects history message

- CLI: After reload, refreshes self.agent.tools and valid_tool_names
  so the model sees updated tools on its next API call
- Both CLI and Gateway: Appends a [SYSTEM: ...] message at the END
  of conversation history explaining what changed (added/removed/
  reconnected servers, tool count). This preserves prompt-cache for
  the system prompt and earlier messages — only the tail changes.
- Gateway already creates a new AIAgent per message so tools refresh
  naturally; the injected message provides context for the model

gateway/run.py

@@ -1422,6 +1422,31 @@ class GatewayRunner:
                 lines.append("No MCP servers connected.")
             else:
                 lines.append(f"\n🔧 {len(new_tools)} tool(s) available from {len(connected_servers)} server(s)")
+
+            # Inject a message at the END of the session history so the
+            # model knows tools changed on its next turn.  Appended after
+            # all existing messages to preserve prompt-cache for the prefix.
+            change_parts = []
+            if added:
+                change_parts.append(f"Added servers: {', '.join(sorted(added))}")
+            if removed:
+                change_parts.append(f"Removed servers: {', '.join(sorted(removed))}")
+            if reconnected:
+                change_parts.append(f"Reconnected servers: {', '.join(sorted(reconnected))}")
+            tool_summary = f"{len(new_tools)} MCP tool(s) now available" if new_tools else "No MCP tools available"
+            change_detail = ". ".join(change_parts) + ". " if change_parts else ""
+            reload_msg = {
+                "role": "user",
+                "content": f"[SYSTEM: MCP servers have been reloaded. {change_detail}{tool_summary}. The tool list for this conversation has been updated accordingly.]",
+            }
+            try:
+                session_entry = self.session_store.get_or_create_session(event.source)
+                self.session_store.append_to_transcript(
+                    session_entry.session_id, reload_msg
+                )
+            except Exception:
+                pass  # Best-effort; don't fail the reload over a transcript write
+
             return "\n".join(lines)
 
         except Exception as e: