fix(agents): replace active polling with notification-based waiting for background tasks

Sisyphus prompt instructed 'your next action is background_output' which
caused agents to repeatedly poll running tasks instead of ending their
response and waiting for the system notification.

- Replace 'STOP all other output' with 'end your response' (actionable)
- Add system-reminder notification mechanism explanation
- Add explicit 'Do NOT poll' prohibition
- Reduce background_cancel(all=true) mentions from 5x to 1x (Hard Blocks)
- Reduce Oracle collect obligation from 4x to 2x
- Remove motivational fluff ('blind spots', 'normal and expected')

Net: -2 lines, clearer mechanism, eliminates polling loop root cause.

src/agents/sisyphus.ts

@@ -329,7 +329,7 @@ task(subagent_type="explore", run_in_background=true, load_skills=[], descriptio
 // Reference Grep (external)
 task(subagent_type="librarian", run_in_background=true, load_skills=[], description="Find JWT security docs", prompt="I'm implementing JWT auth and need current security best practices to choose token storage (httpOnly cookies vs localStorage) and set expiration policy. Find: OWASP auth guidelines, recommended token lifetimes, refresh token rotation strategies, common JWT vulnerabilities. Skip 'what is JWT' tutorials — production security guidance only.")
 task(subagent_type="librarian", run_in_background=true, load_skills=[], description="Find Express auth patterns", prompt="I'm building Express auth middleware and need production-quality patterns to structure my middleware chain. Find how established Express apps (1000+ stars) handle: middleware ordering, token refresh, role-based access control, auth error propagation. Skip basic tutorials — I need battle-tested patterns with proper error handling.")
-// Continue working immediately. Collect with background_output when needed.
+// Continue working immediately. System notifies on completion — collect with background_output then.
 
 // WRONG: Sequential or blocking
 result = task(..., run_in_background=false)  // Never wait synchronously for explore/librarian
@@ -337,10 +337,10 @@ result = task(..., run_in_background=false)  // Never wait synchronously for exp
 
 ### Background Result Collection:
 1. Launch parallel agents \u2192 receive task_ids
-2. Continue immediate work (explore, librarian results)
-3. When results needed: \`background_output(task_id="...")\`
-4. **If Oracle is running**: STOP all other output. Follow Oracle Completion Protocol in <Oracle_Usage>.
-5. Cleanup: Cancel disposable tasks (explore, librarian) individually via \`background_cancel(taskId="...")\`. Never use \`background_cancel(all=true)\`.
+2. Continue immediate work
+3. System sends \`<system-reminder>\` on each task completion — then call \`background_output(task_id="...")\`
+4. Need results not yet ready? **End your response.** The notification will trigger your next turn.
+5. Cleanup: Cancel disposable tasks individually via \`background_cancel(taskId="...")\`
 
 ### Search Stop Conditions
 
@@ -477,9 +477,8 @@ If verification fails:
 3. Report: "Done. Note: found N pre-existing lint errors unrelated to my changes."
 
 ### Before Delivering Final Answer:
-- **If Oracle is running**: STOP. Follow Oracle Completion Protocol in <Oracle_Usage>. Do NOT deliver any answer.
-- Cancel disposable background tasks (explore, librarian) individually via \`background_cancel(taskId="...")\`.
-- **Never use \`background_cancel(all=true)\`.**
+- If Oracle is running: **end your response** and wait for the completion notification first.
+- Cancel disposable background tasks individually via \`background_cancel(taskId="...")\`.
 </Behavior_Instructions>
 
 ${oracleSection}