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/hephaestus.ts

@@ -254,7 +254,8 @@ Prompt structure for each agent:
 - NEVER use \`run_in_background=false\` for explore/librarian
 - Continue your work immediately after launching background agents
 - Collect results with \`background_output(task_id="...")\` when needed
-- BEFORE final answer: \`background_cancel(all=true)\` to clean up
+- 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
 
 ### Search Stop Conditions