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/dynamic-agent-prompt-builder.ts

@@ -277,12 +277,11 @@ Briefly announce "Consulting Oracle for [reason]" before invocation.
 
 ### Oracle Background Task Policy:
 
-**You MUST collect Oracle results before your final answer. No exceptions.**
+**Collect Oracle results before your final answer. No exceptions.**
 
-- Oracle may take several minutes. This is normal and expected.
-- When Oracle is running and you finish your own exploration/analysis, your next action is \`background_output(task_id="...")\` on Oracle — NOT delivering a final answer.
-- Oracle catches blind spots you cannot see — its value is HIGHEST when you think you don't need it.
-- **NEVER** cancel Oracle. **NEVER** use \`background_cancel(all=true)\` when Oracle is running. Cancel disposable tasks (explore, librarian) individually by taskId instead.
+- Oracle takes minutes. When done with your own work: **end your response** — wait for the \`<system-reminder>\`.
+- Do NOT poll \`background_output\` on a running Oracle. The notification will come.
+- Never cancel Oracle.
 </Oracle_Usage>`
 }
 
@@ -292,8 +291,8 @@ export function buildHardBlocksSection(): string {
     "- Commit without explicit request — **Never**",
     "- Speculate about unread code — **Never**",
     "- Leave code in broken state after failures — **Never**",
-    "- `background_cancel(all=true)` when Oracle is running — **Never.** Cancel tasks individually by taskId.",
-    "- Delivering final answer before collecting Oracle result — **Never.** Always `background_output` Oracle first.",
+    "- `background_cancel(all=true)` — **Never.** Always cancel individually by taskId.",
+    "- Delivering final answer before collecting Oracle result — **Never.**",
   ]
 
   return `## Hard Blocks (NEVER violate)
@@ -308,8 +307,8 @@ export function buildAntiPatternsSection(): string {
     "- **Testing**: Deleting failing tests to \"pass\"",
     "- **Search**: Firing agents for single-line typos or obvious syntax errors",
     "- **Debugging**: Shotgun debugging, random changes",
-    "- **Background Tasks**: `background_cancel(all=true)` — always cancel individually by taskId",
-    "- **Oracle**: Skipping Oracle results when Oracle was launched — ALWAYS collect via `background_output`",
+    "- **Background Tasks**: Polling `background_output` on running tasks — end response and wait for notification",
+    "- **Oracle**: Delivering answer without collecting Oracle results",
   ]
 
   return `## Anti-Patterns (BLOCKING violations)

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}