Rename "affinity-assistant" to "coschedule-assistant" in user-visible surfaces

[vdemeester]

Summary

The disable-affinity-assistant feature flag was renamed to coschedule (see #7307), but the underlying pods, StatefulSets, labels, annotations, and ConfigMap keys still use the old "affinity-assistant" naming. This is confusing — users see coschedule in the config but affinity-assistant in their cluster.

We should rename the user-visible surfaces from "affinity-assistant" to "coschedule-assistant" to align with the feature flag.

User-Visible Surfaces to Rename

#	What	Current Value	Proposed Value
1	Container name	affinity-assistant	coschedule-assistant
2	StatefulSet name prefix	affinity-assistant-<hash>	coschedule-assistant-<hash>
3	Label app.kubernetes.io/component	affinity-assistant	coschedule-assistant
4	Label app.kubernetes.io/instance	affinity-assistant-<hash>	coschedule-assistant-<hash>
5	Annotation on TaskRun pods	pipeline.tekton.dev/affinity-assistant	pipeline.tekton.dev/coschedule-assistant
6	ConfigMap key	default-affinity-assistant-pod-template	default-coschedule-assistant-pod-template
7	Event reason	ReasonCouldntCreateOrUpdateAffinityAssistantStatefulSet	ReasonCouldntCreateOrUpdateCoscheduleAssistantStatefulSet

Proposed Phased Approach

Phase 1: Accept both names (non-breaking)

• New StatefulSets/pods use coschedule-assistant prefix
• Cleanup code looks for both old and new StatefulSet names (handles in-flight PipelineRuns during upgrade)
• ConfigMap accepts both default-affinity-assistant-pod-template and default-coschedule-assistant-pod-template
• Write both annotations on TaskRun pods, read either
• Label selectors match on either component name

Phase 2: Deprecation warnings (1-2 releases later)

• Log deprecation warnings when old ConfigMap key / annotation is detected
• Documentation updated to reference new names only

Phase 3: Remove old names (2-3 releases after Phase 1)

• Drop old constants and dual-read/write logic
• Only accept new names

Key Files Affected

• pkg/workspace/affinity_assistant_names.go — constants for labels/annotations
• pkg/reconciler/pipelinerun/affinity_assistant.go — StatefulSet creation, naming, cleanup
• pkg/apis/config/default.go — ConfigMap key for default pod template
• pkg/reconciler/pipelinerun/pipelinerun.go — annotation on TaskRun pods
• pkg/reconciler/taskrun/taskrun.go — reads annotation

Internal Go types and file names (e.g., AffinityAssistantTemplate, affinity_assistant.go) can be renamed separately in a follow-up to keep this focused on user-visible changes.

Why

• Consistency: the feature flag is coschedule, but the artifacts in the cluster say affinity-assistant
• Clarity: "coschedule-assistant" better describes what the pod does (co-schedules tasks to the same node)
• The old disable-affinity-assistant flag has already been removed and migrated to coschedule

/kind feature