Add stale issue/PR bot workflow

[HeyItsGilbert]

Depends on

#157 — complete manual triage before enabling this, so the bot doesn't sweep the existing backlog.

What

Create .github/workflows/stale.yml with the following content:

name: Mark stale issues and PRs

on:
  schedule:
    - cron: '0 0 * * *'

jobs:
  stale:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/stale@v9
        with:
          stale-issue-message: 'This issue has had no activity for 60 days. It will be closed in 30 days unless there is new activity or a maintainer removes the stale label.'
          stale-pr-message: 'This PR has had no activity for 60 days. It will be closed in 30 days unless there is new activity or a maintainer removes the stale label.'
          close-issue-message: 'Closing due to inactivity. Please re-open if this is still relevant against the current version.'
          days-before-stale: 60
          days-before-close: 30
          stale-issue-label: 'status: stale'
          stale-pr-label: 'status: stale'
          exempt-issue-labels: 'priority: high'

Why

Automates backlog hygiene going forward. Safe to enable only after #157 clears the existing stale issues manually.