## Summary
- Add `device_has_effective_agent?/1` to Agents context that checks full cascade (device → site → org → global default) without distinguishing between cloud pollers and local agents
- Update check_executor_worker, device_poller_worker, and device_monitor_worker to use new function
- Phoenix never executes checks (DNS, ping, HTTP, TCP, SSL, SNMP) when any agent handles the device
## Problem
The global default cloud poller was not being checked in `should_phoenix_poll_device?/1`, causing Phoenix to run DNS/ping/service checks server-side even when the Go agent was handling the device. This produced duplicate results with NXDOMAIN errors from the Erlang DNS resolver.
## Test plan
- [x] 6 new tests for `device_has_effective_agent?/1` covering all cascade levels
- [x] Full test suite passes (8601 tests)
- [ ] Verify staging DNS check stops showing NXDOMAIN errors after deploy
Reviewed-on: graham/towerops-web#77
Graham McIntire
f3cf1a9b3c