[2.1] Add stale issue management workflow

[vbonk]

What needs to be done

Create .github/workflows/stale.yml using actions/stale (SHA-pinned). workflow_dispatch ONLY in template (schedule commented). 60d stale, 14d close. Exempt: status:in-progress, status:blocked, priority:high, roadmap.

Acceptance Criteria

• stale.yml exists with workflow_dispatch trigger
• Schedule commented out
• Exemptions for existing label taxonomy

[vbonk]

Completed in Phase 2 PR #86.