fix(prompt): prevent immediate BackgroundOutput polling after background task launch

Tool return values contained CTAs ("Use background_output to check") that
caused models to immediately poll instead of waiting for <system-reminder>
notifications. Unified all 5 tool return messages with explicit "Do NOT call
background_output now" guard, added pre-notification warning to BackgroundOutput
tool description, and strengthened Background Result Collection sections across
all 3 Sisyphus prompt variants (default, gpt-5-4, main) with BLOCKING
anti-pattern enforcement.

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/tools/delegate-task/background-task.ts

@@ -149,7 +149,9 @@ Description: ${task.description}
 Agent: ${task.agent}${args.category ? ` (category: ${args.category})` : ""}
 Status: ${task.status}
 
-System notifies on completion. Use \`background_output\` with task_id="${task.id}" to check.${taskMetadataBlock}`
+System notifies on completion. Use \`background_output\` with task_id="${task.id}" to check.
+
+Do NOT call background_output now. Wait for <system-reminder> notification first.${taskMetadataBlock}`
   } catch (error) {
     return formatDetailedError(error, {
       operation: "Launch background task",