Dev keeps auto-deploying on every push to main, same as before, but now
also publishes each build to git.jerodrigged.com's container registry
tagged by short commit SHA (best-effort - never blocks the dev deploy
if REGISTRY_TOKEN isn't set yet).
Prod deploys only on an intentional `git push origin main:prod`, and
only ever pulls a pre-built SHA-tagged image - it never rebuilds from
source. This guarantees prod runs the exact artifact dev already
validated, and makes promoting an untested commit fail loudly (pull of
a nonexistent tag) instead of silently rebuilding something new.
Needs new secrets before the prod path works: PROD_SSH_KEY, PROD_HOST,
PROD_USER, REGISTRY_USER, REGISTRY_TOKEN. PROD_SSH_KEY/PROD_HOST/
REGISTRY_USER are already set; PROD_USER and REGISTRY_TOKEN still need
Jerod's input.
