Fills in the result summary on the task from executor response.
Single pass: find section "Result summary" → build one synthetic exec run with that content. If no "Result summary" section, appends to issues and returns null. Does not mutate Details.tools or set Details.exec_done; caller applies the returned Tool when appropriate. Used by legacy/test paths; normal execution uses exec_extract(Tool) and the execution queue.
Content we expect (task_execution.md):
## Result summary
(Substance + whether needs are met; gaps or follow-up in prose as needed)| task |
the task (runner/session context for the synthetic Tool) |
|
the synthetic run, or null when the required section is missing |