selfhost: the documented image pull is by mutable tag, not digest

[L4XB]

Problem

README.md and deploy/community/README.md both document the fast start as:

RELEASE=v0.2.0-alpha.1
export SIX_API_IMAGE=ghcr.io/sixsentences/community-api:$RELEASE
export SIX_WEB_IMAGE=ghcr.io/sixsentences/community-web:$RELEASE-localhost

A registry tag is a mutable pointer. Anyone who can push to the registry can
move v0.2.0-alpha.1 to different bytes, and every operator who pulls
afterwards silently gets them. The Git tag it names is immutable by ruleset;
the image tag is not.

This matters more here than in most projects, because the whole release design
is built around immutability — signed Git tags, verified ancestry, published
checksums, build provenance for the Python artifacts. The images are the one
link in that chain an operator cannot verify.

What to consider

1. Publish the digest. publish-images.yml already prints it
   (v0.2.0-alpha.1-localhost: digest: sha256:20109bb…). Recording it on the
   release page, or in the repository at the tag, would let an operator pin
   image@sha256:… and know what they got.
2. Attest the image digests, the way the Python artifacts already are. This
   overlaps release: produce reproducible full-stack images with SBOM and provenance #19, which owns the wider question — coordinate rather than
   duplicate.
3. Say it plainly meanwhile. Until digests are published, the documentation
   should not imply the pull is as verifiable as the rest of the release.

Acceptance criteria

• An operator can pin by digest, and the digest comes from a source they can
  check
• The documented quick start stays a quick start — pinning may be the
  hardened variant rather than the default
• The relationship to release: produce reproducible full-stack images with SBOM and provenance #19 is stated, so the two do not implement it twice