Add cancellation check after tool execution in harness chat loop

This allows the user to interrupt between rapid tool calls. When Ctrl+C is
pressed during a sequence of tool calls, the harness now stops processing
rather than completing the entire batch.

The check follows the same semantics as the existing cancellation checks:
returns empty string (success) to preserve conversation state in the chat file.

src/harness.cpp

@@ -161,6 +161,12 @@ std::expected<std::string, std::string> Harness::chat(const Message& user_messag
                 tool_result.content = result;
                 push_message(tool_result);
             }
+
+            if (ctrlc_request)
+                // Cancellation after tool execution: same semantics as above -
+                // return empty string to preserve conversation state in the chat file.
+                return "";
+
             continue;
         }