docs: add Background Tasks page and clean up cross-references

New page: docs/automation/tasks.md — comprehensive reference for the task
system covering lifecycle, delivery, notifications, audit, CLI commands,
storage, maintenance, and how tasks relate to cron/heartbeat/sessions.

- Add to Mintlify navigation (docs.json) under Automation group
- Clean up engineer's earlier scattered additions in cron-jobs.md,
  cron-vs-heartbeat.md, and heartbeat.md to be concise and link to the
  new canonical tasks page
- Replace verbose inline explanations with cross-reference links

docs/docs.json

@@ -1118,6 +1118,7 @@
                   "automation/standing-orders",
                   "automation/cron-jobs",
                   "automation/cron-vs-heartbeat",
+                  "automation/tasks",
                   "automation/troubleshooting",
                   "automation/webhook",
                   "automation/gmail-pubsub",