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/create-background-task.ts

@@ -114,10 +114,9 @@ Description: ${task.description}
 Agent: ${task.agent}
 Status: ${task.status}
 
-The system will notify you when the task completes.
-Use \`background_output\` tool with task_id="${task.id}" to check progress:
-- block=false (default): Check status immediately - returns full status info
-- block=true: Wait for completion (rarely needed since system notifies)`
+System notifies on completion. Use \`background_output\` with task_id="${task.id}" to check.
+
+Do NOT call background_output now. Wait for <system-reminder> notification first.`
       } catch (error) {
         const message = error instanceof Error ? error.message : String(error)
         return `[ERROR] Failed to launch background task: ${message}`