refactor(triage): mark stale closures with stable metadata #5487

User Story

As an OpenShell maintainer reviewing labeling automation, I need tools to distinguish stale closures from other automated closures without depending on comment wording or configured day counts.

Description

Add a stable metadata marker to the staleness workflow's closing comment and centralize its validation in an importable is_stale_closed helper. A marker must be paired with the expected bot identity and matching closure metadata; copied text alone must not establish provenance.

Context

The workflow is .github/workflows/stale.yml. The native check in #5355 uses the current state:stale label, a not_planned closure, and matching bot/event metadata. It identifies the actor but cannot distinguish which GitHub Actions workflow closed the issue.

Related: #5355 and #5276.

Definition of Done

• Stale issue closures emit a stable machine-readable marker.
• Labeling tools use the shared predicate to recognize marked stale closures.
• Tests cover valid closures, missing/copied markers, mismatched actors/events, and changes to message wording or day counts.
• Handling of existing unmarked closures is explicit.