Migrate CI to shipshape standard

[ivoronin]

Aligns CI with the consolidated github-workflows model:

• Remove the dangling automerge.yml caller — the reusable automerge.yml was deleted; Dependabot PRs are now merged manually.
• SHA-pin the github-workflows reusable calls (@<sha> # main) in test.yml/release.yml.
• Dependabot: add the github-actions ecosystem (keeps the SHA pins current).

Handled separately as repo settings: disable auto-merge, enable Dependabot security updates, remove the orphan PAT_TOKEN.

Note: tag the next release vYYYY.0M.0D (with the v prefix) so it matches release.yml's v* trigger — the latest tag 2025.12.11 lacks it.