External triggers and Run setup

Studio Cron tab  →  prefill from canvas + saved defaults (UI only)
Save schedule    →  store start params on workflow_triggers.config
Cron fire        →  engine uses trigger.config + graph only

~~effective params = merge(saved run_setup_json, caller override)~~

{
  "trigger_payload": { "source": "shopify" },
  "trigger_steps": { "uuid-root": { "limit": 10 } },
  "excluded_step_ids": []
}

{
  "cron": "0 9 * * *",
  "timezone": "UTC",
  "trigger_steps": {
    "uuid-mcp-root": { "limit": 30, "rss_feed_url": "https://hnrss.org/frontpage" }
  },
  "excluded_step_ids": ["uuid-agent-root"]
}

{
  "use_run_setup_defaults": true,
  "body": { "topic": "weekly-digest" }
}

{
  "command": "start",
  "params": {
    "trigger_type": "api",
    "trigger_payload": { "topic": "api-only" },
    "trigger_steps": { "uuid-root": { "limit": 5 } },
    "excluded_step_ids": []
  }
}