use mergify to auto-merge dependabot changes

[ktdreyer]

.github/workflows/dependabot-auto-merge.yml is the only job that uses the sketchy pull_request_target trigger.

After landing basic mergify support #858 , try replacing this homegrown task with https://docs.mergify.com/workflow/actions/review/#approve-dependabot-pull-requests

[github-actions]

Inactive for 30 days. Will close in 7 days unless there's activity.

[jeremyeder]

Housekeeping: reviewing open issues. @ktdreyer is this still relevant / something you plan to work on? If not, we'll close it out. No pressure either way — just trimming the backlog.

[github-actions]

🤖 Amber is working on this issue. A PR will be created shortly with the ambient-code:managed label.

[github-actions]

🤖 Amber session completed (phase: Running). View session