ALL-1381: Spec shadow monitors and trip timeline details

Draft a reviewable spec for shadow monitors from Slack thread 1780506881.863879. V1: no audience model; evaluator runs normally; monitor states/transitions persist; no customer-facing Alert rows or destination fanout (email/SMS/webhook/etc.); monitor detail becomes Datadog-like trip timeline/detail surface. Inspect current MonitorDetail transition payload/context and specify any added fields. Open PR with spec and assign sephcoster.