chore(harness): add actions/stale workflow to auto-close inactive issues and PRs

[keli-wen]

Summary

Add GitHub's maintained actions/stale action so inactive issues and pull requests are surfaced and eventually closed automatically, keeping the backlog honest. The job runs daily at 01:00 UTC and can also be triggered manually via workflow_dispatch.

The policy is two-phase (warn before close), so nothing is closed cold:

• An issue or PR with no activity for 14 days is marked stale with a comment.
• If it stays inactive for 7 more days it is closed (21 days total).
• Any comment, commit, or label change resets the clock and clears the stale label.
• help wanted and good first issue are exempt; draft PRs are never marked stale.

The stale label is managed by the action; it is an operational state label, separate from the type: / area: / impact: taxonomy in contexts/dev/labels.md.

Related Issue

No issue applies — this is repository maintenance automation.

Verification

• check-yaml pre-commit hook validated the workflow file.
• This is a workflow-only change with no Python diff, so the verify pre-push hook (scoped to types: [python]) is not exercised and scripts/verify.sh has no runtime surface to cover here.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes. — Not exercised: no Python diff; the verify hook is Python-scoped.
• Every applicable live-network component smoke test passes, or this PR states why none applies. — None applies; no component code changes.
• Public behavior has focused tests, an example, and documentation where applicable. — Not applicable to a CI-only workflow.
• The PR is complete, small, and contains no unrelated changes.