Move Dependabot automerge behind completed required checks

[Copilot]

Dependabot automerge was running inside the same workflow that defines the required checks, so the merge action executed before those checks could become satisfied. This changes the merge trigger to run only after the Build and Test workflow completes successfully for a Dependabot PR.

• Problem

  • automerge lived in build-and-test.yml, which made it part of the required-check set it was waiting on.
  • The result was a deadlock at merge time: 3 of 3 required status checks are expected.

• Workflow split

  • Remove the automerge job from Build and Test.
  • Add a dedicated Dependabot automerge workflow triggered by workflow_run for the Build and Test workflow.

• Merge gating

  • Run automerge only when the upstream workflow:
    • was triggered by pull_request
    • completed with success
    • was started by dependabot[bot]
    • is associated with a PR

• PR targeting

  • Pass the PR number from the completed workflow run into fastify/github-action-merge-dependabot, so merge operates on the already-validated Dependabot PR rather than the in-flight workflow context.

on:
  workflow_run:
    workflows: [Build and Test]
    types: [completed]

jobs:
  automerge:
    if: >
      github.event.workflow_run.event == 'pull_request' &&
      github.event.workflow_run.conclusion == 'success' &&
      github.event.workflow_run.actor.login == 'dependabot[bot]'