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/background-task/constants.ts

@@ -2,6 +2,8 @@ export const BACKGROUND_TASK_DESCRIPTION = `Run agent task in background. Return
 
 Use \`background_output\` to get results. Prompts MUST be in English.`
 
-export const BACKGROUND_OUTPUT_DESCRIPTION = `Get output from background task. Use full_session=true to fetch session messages with filters. System notifies on completion, so block=true rarely needed. - Timeout values are in milliseconds (ms), NOT seconds.`
+export const BACKGROUND_OUTPUT_DESCRIPTION = `Get output from background task. Use full_session=true to fetch session messages with filters. System notifies on completion, so block=true rarely needed. - Timeout values are in milliseconds (ms), NOT seconds.
+
+IMPORTANT: ONLY call this tool AFTER receiving a <system-reminder> notification for the task. Do NOT call immediately after launching a background task - wait for the notification first.`
 
 export const BACKGROUND_CANCEL_DESCRIPTION = `Cancel running background task(s). Use all=true to cancel ALL before final answer.`