docs: document background jobs across prompts, config example, and README

- Extend the injected Background Jobs prompt guidance: system_notifications
  push on completion, collect-only-when-idle wait protocol, and the graph
  LLM-node collect-before-final-turn rule
- Mention the system_notifications push in the agent spawning guidance and
  in the sisyphus/architect wait-protocol text (agent completions push
  notifications too)
- config.example.yaml: max_concurrent_jobs (default 5, 0 = disabled)
- README: features-list entry pointing at the Background-Jobs wiki page

assets/agents/architect/config.yaml

@@ -261,7 +261,7 @@ instructions: |
 
   3. **Wait for Sisyphus.** Do not poll `agent__collect` on a running agent — do non-overlapping work
      (e.g. prep the next task's context) or end your response and wait for the completion
-     notification, then `agent__collect`.
+     notification (a `system_notifications` entry on your next tool result), then `agent__collect`.
 
   4. **Verify against the plan (divergence check).** When Sisyphus returns, do NOT trust its
      self-report — get an INDEPENDENT conformance verdict:

assets/agents/sisyphus/config.yaml

@@ -233,7 +233,7 @@ instructions: |
 
   1. Do non-overlapping work if any (work that doesn't depend on delegated results).
   2. If none → **end your response.** Do not call `agent__collect` immediately.
-  3. The system notifies you on completion.
+  3. The system notifies you on completion — a `system_notifications` entry appears on your next tool result naming the exact collect command.
   4. On notification, call `agent__collect` to retrieve results.
 
   ### Anti-duplication rule (BLOCKING)