Add workflow to create PRs from the Gardener backlog

[gonzaloriestra]

WHY are these changes introduced?

Turn prioritized Gardener backlog issues into reviewable CLI fixes.

WHAT is this pull request doing?

Adds gardener-backlog-prs to pick one open CLI issue from the Gardener board, ordered P0, P1, P2, and so on, and use Claude to create a draft PR and share its linked title in #devtools-gardener-backlog. Runs daily at 00:00 UTC or manually.

Selection follows view 61's current filter, includes Backlog and Untriaged issues, puts unprioritized work last, and skips prior workflow PRs. Equal priorities use oldest-first order. Reads the board and private issues with SHOP_GH_READ_CONTENT_TOKEN; creates CLI PRs with GITHUB_TOKEN.

How to test your changes?

After merge, use Actions → gardener-backlog-prs → Run workflow. Verify that it selects the highest-priority eligible CLI issue and either creates one draft PR with a Slack notification or exits without a PR when no suitable fix exists.

Validated with 10 selection tests, Actionlint, Prettier, and a workflow simulation covering private issue context, Slack links, no-PR results, and access failures. A read-only query against the real board also passed using local credentials. No live Claude or Slack run was triggered.

Post-release steps

The first Actions run must confirm whether SHOP_GH_READ_CONTENT_TOKEN has Projects read access in shop and Issues read access to shop/issues-develop and shop/issues. If access fails, replace that secret with an appropriately authorized token.

GitHub may require a maintainer to select Approve workflows to run on generated PRs before their CI starts.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add