eval: publish and pin the egress proxy image by registry digest

## Context

PR #2947 builds `maka-eval-egress-proxy:12.2.3` locally from the checked-in Dockerfile and policy. The tag is versioned, and the per-cell health gate plus policy tests fail closed, but a local Docker tag is still mutable and has no portable `RepoDigest` until the image is published to a registry.

## Required follow-up

1. Add a release workflow that builds `packages/eval/harbor/egress-proxy/Dockerfile` from the reviewed source.
2. Publish the image to the project registry for Linux amd64 (and any additional supported architectures).
3. Record the immutable manifest digest in the Eval spec/Compose overlay.
4. Make executor preflight reject a missing or mismatched digest before starting Harbor.
5. Add a regression test proving that a stale same-name local tag cannot satisfy the pinned deployment contract.

## Why separate

A valid digest must identify a published registry manifest. Pinning the current machine-local image ID would be host/build-instance-specific and would not be reproducible for CI or another VMSS node.