Replace Prow automated PR plugins with lightweight tasks or services

[vdemeester]

Summary

Several Prow plugins run automatically on PR events (no slash command needed)
to add labels or validate PR content. These need replacements — either as
lightweight Tekton tasks triggered via PaC / EventListener on the OCI cluster,
or as simple GHA workflows.

Plugins to Replace

Plugin	Trigger	What it does
size	PR open/update	Adds size/XS..size/XXL labels based on diff size
verify-owners	PR open/update	Validates OWNERS files syntax and content
wip	PR open/update	Adds do-not-merge/work-in-progress when title contains WIP
release-note	PR open/update	Validates release note block, manages release-note-* labels
blunderbuss	PR open	Auto-assigns reviewers based on OWNERS files
needs-rebase	PR update	Adds needs-rebase label when PR has merge conflicts

Considerations

• wip could be replaced by GitHub's native draft PR feature — evaluate if
  we still need the label at all.
• blunderbuss uses OWNERS files; GitHub's native CODEOWNERS is an alternative
  but has different semantics. Need to decide if we converge on CODEOWNERS or
  keep OWNERS.
• verify-owners is important as long as we keep OWNERS files for /approve
  and /lgtm. If we switch to CODEOWNERS, it becomes unnecessary.
• size is nice-to-have. Many lightweight GHA actions exist for this.
• needs-rebase — GitHub branch protection "require up-to-date" is the native
  equivalent but has different UX.
• Good candidates for either reusable GHA workflows shared from plumbing, or
  PaC-triggered Tekton tasks on the OCI cluster.

Related

• Replace Tide merge queue with GitHub Merge Queue #3177 — Tide → GitHub Merge Queue (some of these labels gate merging)
• Implement label check for "kind/*" using GHA #2841 — Implement label check for kind/* using GHA (related work)

/cc @tektoncd/plumbing-maintainers