Filter PromotionStrategy watch events for DependentsSuccessfulCommitStatus reconciler

[crenshaw-dev]

Problem

In 0.38, DependentsSuccessfulCommitStatusReconciler watches PromotionStrategy without a predicate:

Watches(&promoterv1alpha1.PromotionStrategy{}, r.enqueueDependentsSuccessfulCommitStatusForPromotionStrategy()).

Every PromotionStrategy informer event enqueues matching DSCS resources — including status-only updates that do not affect gate evaluation (Ready condition changes, observedGeneration, instanceID, environment history, etc.).

In practice, most DSCS reconciles are no-ops: every environment has active.dry.sha == proposed.dry.sha, so the controller skips CommitStatus writes and only refreshes status.environments. Those reconciles still perform a PS Get, DAG walk, and DSCS status SSA patch.

This adds significant reconcile volume on busy clusters where PromotionStrategy status changes frequently (e.g. from the existing CTP → PS feedback via Owns(ChangeTransferPolicy)).

Impact

• High DSCS reconcile count driven by PS status churn, mostly no-op work.
• Controller contention (CPU, API server) that can inflate reconcile durations for other controllers in the same process.
• Not a cross-controller cascade: no-op DSCS reconciles do not write CommitStatus resources and nothing watches DSCS, so they do not directly re-queue PromotionStrategy or ChangeTransferPolicy. The PS ↔ CTP loop remains the primary driver of PS/CTP reconcile frequency.

Proposal

Add a field-selective predicate on the PromotionStrategy watch, similar to existing patterns in the codebase:

• applicationPredicate on Argo CD Application watches (only health/sync/revision changes)
• promotionStrategyCreatePredicate on ArgoCDCommitStatus (create-only — DSCS needs updates, but not all of them)

Enqueue DSCS when gate-relevant PromotionStrategy state changes:

Spec

• spec.environments (affects inferred linear DAG when DSCS spec.environments is omitted)

Status (per environment)

• branch
• active.dry.sha, active.dry.commitTime
• proposed.dry.sha, proposed.hydrated.sha
• proposed.note.drySha (used by getEffectiveHydratedDrySha)
• active.commitStatuses[].key and .phase (upstream health via checkCommitStatusesPassing)

Always enqueue on PromotionStrategy create and delete.

Sketch

Watches(
    &promoterv1alpha1.PromotionStrategy{},
    r.enqueueDependentsSuccessfulCommitStatusForPromotionStrategy(),
    builder.WithPredicates(promotionStrategyGateRelevantPredicate()),
)

The predicate UpdateFunc should compare a gate-relevant projection of old vs new status.environments (and spec.environments), not raw object equality.

Testing

• Unit tests for the predicate: ignore Ready/history-only status changes; allow SHA, note, and commit-status phase transitions.
• Controller test: PS status patch that does not change gate-relevant fields does not enqueue DSCS; upstream health phase flip does enqueue.

Related code

• internal/controller/dependentssuccessfulcommitstatus_controller.go — SetupWithManager, updateDependentsSuccessfulCommitStatus, isUpstreamPending
• internal/controller/argocdcommitstatus_controller.go — applicationPredicate, promotionStrategyCreatePredicate as precedents