[2.11] Add lock old threads workflow

[vbonk]

What needs to be done

Create .github/workflows/lock-threads.yml using dessant/lock-threads (SHA-pinned). Locks issues/PRs closed 30+ days. Weekly schedule. Active by default.

Acceptance Criteria

• Workflow exists with weekly schedule
• 30-day threshold
• Locks both issues and PRs

[vbonk]

Completed in Phase 2 PR #86.