refactor: break up appHealthOrSyncStatusChanged into shared predicates

[EronWright]

Context

#5995 introduced appHealthOrSyncStatusChanged() in
pkg/controller/stages/event_handlers.go — a monolithic function that
detects four distinct signals and gates the Stage reconciler's Application
watch:

1. Health status change
2. Operation phase change (any transition)
3. Sync revision change
4. reconciledAt crossing finishedAt (health becomes trustworthy after a hard refresh)

#5832 added typed predicates for the Promotions controller's Application watch:
ArgoCDAppOperationCompleted, ArgoCDAppHealthChanged, ArgoCDAppSyncChanged,
and ArgoCDAppReconciledAfterOperation. Signals 1, 2, and 4 are identical in
both controllers; only signal 3 (sync revision) is Stages-specific.

Proposed refactor

Break appHealthOrSyncStatusChanged() into three exported typed predicates
(parallel to those in pkg/controller/promotions/predicates.go) that can be
shared:

• ArgoCDAppHealthChanged — signal 1
• ArgoCDAppOperationPhaseChanged — signal 2 (any phase change, not just to-completed)
• ArgoCDAppReconciledAfterOperation — signal 4

Move them to a shared location (e.g. pkg/controller/argocd/predicates.go)
and use them in both the Stages and Promotions watches, replacing the current
inline implementations in each.

Signal 3 (sync revision) remains Stages-only and stays in
event_handlers.go.

Reference

• Discussed in argocd-wait review: feat: add argocd-wait promotion step #5832 (comment)
• Kent's follow-up note: feat: add argocd-wait promotion step #5832 (comment)