fix(agents): replace background_cancel(all=true) with individual task cancellation

Atlas and Sisyphus prompts instructed agents to use background_cancel(all=true)
before final answers. This destroys uncollected background task results and
contradicts existing NEVER directives in the Sisyphus prompt, causing agents
to lose explore/librarian outputs mid-session.

Replace with individual task cancellation pattern that preserves completed
task results while still cleaning up running disposable tasks.

src/agents/atlas/default.ts

@@ -311,7 +311,8 @@ task(category="quick", load_skills=[], run_in_background=false, prompt="Task 4..
 
 **Background management**:
 - Collect results: \`background_output(task_id="...")\`
-- Before final answer: \`background_cancel(all=true)\`
+- Before final answer, cancel DISPOSABLE tasks individually: \`background_cancel(taskId="bg_explore_xxx")\`, \`background_cancel(taskId="bg_librarian_xxx")\`
+- **NEVER use \`background_cancel(all=true)\`** — it kills tasks whose results you haven't collected yet
 </parallel_execution>
 
 <notepad_protocol>