Replace "draft trick" documentation with auto-merge flow

Since GitHub's June 11, 2026 change "Bot-created pull requests can run
workflows if approved"[1], workflow runs triggered by the changelog
commit pushed with `GITHUB_TOKEN` are created in an approval-required
state instead of being dropped. A maintainer can start the required
checks with the "Approve workflows to run" button, so the "draft trick"
documented in #476 is no longer necessary and the reusable workflow
enables "auto-merge" on the PR again.

- Update the `process-dependabot-reusable.yaml` documentation to
  describe the new flow.
- Remove the `ready_for_review` trigger from `build.yaml` and
  `codeql-analysis.yaml`, restoring the default `pull_request` types.

[1] https://github.blog/changelog/2026-06-11-bot-created-pull-requests-can-run-workflows-if-approved/

Assisted-By: Claude Fable 5 <noreply@anthropic.com>