feat(release): pilot trunk-based CI/CD pipeline

[JSisques]

Summary

Pilots the org-wide trunk-based CI/CD proposal (sisques-labs/workflows#72, implementation in #73) in beacon-api, now that develop/staging are merged into main and beacon-api is the first bounded-context template consumer.

• Replaces release-train.yml with trunk-ci-cd.yml: every push to main builds one commit-addressed image (:sha-<shortsha>, :edge — no version bump) and sequences it through deploy-dev → deploy-pre.
• release.yml no longer rebuilds from source. It now takes a source_digest input and uses docker-release.yml's new bump_mode: promote to retag an already-validated digest as the versioned stable release — the image that ships to prod is byte-for-byte what was validated in pre.
• Only one channel remains (stable); alpha/beta/rc are gone along with develop/staging.
• Updated README.md and docker/README.md to drop stale develop/staging/prerelease references.

Notes

• Temporarily pinned to sisques-labs/workflows@feat/trunk-based-ci-cd-01-pipeline in both workflow files (marked with TODO(trunk-based-ci-cd)) since trunk-ci-cd.yml and bump_mode: promote don't exist on that repo's main yet. Must be re-pinned to @main once workflows#72/fix(deps): update dependency @apollo/server to v5 #73 merge.
• deploy-dev/deploy-pre are placeholders (log-only) — no real dev/pre infrastructure exists yet. GitHub auto-creates the dev/pre environments on first run (public repo, no protection rules needed to unblock this).
• Checked branch protection and GitHub Environments on this repo: none configured on any branch today, so there's nothing to migrate off of develop/staging — if you want a required-reviewer gate on pre or prod, that's a separate Settings change.
• The unvalidated risk flagged in workflows#73 (multi-arch imagetools create promotion) still needs a real dry-run before the first genuine prod release goes through release.yml.

Test plan

• actionlint clean on both changed/added workflow files
• Pre-push hook (full nest build + vitest) passed
• End-to-end dry run once workflows#72/fix(deps): update dependency @apollo/server to v5 #73 merge and this PR repoints to @main