docs: add Monitor architecture decision record

[somethingwithproof]

Summary

Adds the initial proposed ADR for an incremental, PHP 7.4-compatible migration of Monitor toward a hexagonal architecture.

Covers

• domain, application-service, port, and adapter boundaries
• separate availability, breach, Threshold trigger, acknowledgement, mute, and delivery state
• compatibility with existing Cacti 1.2 hooks and host.monitor_* fields
• a staged migration path with test seams

Impact

Documentation only; no current Monitor behavior changes.