Bump opencode to 1.14.31 and split multi-arch publish across runners

The v1.14.30b publish failed on both variants with 'No space left on
device' — arm64 QEMU-emulated layers were stored alongside amd64 on the
same ~40 GB runner, and the mempalace-toolkit bake-in from v1.14.30b
tipped peak disk over the edge during the nodejs dpkg unpack and the
git-lfs layer export.

Refactor docker-publish.yml to the canonical push-by-digest +
manifest-merge pattern: smoke test (amd64) runs on its own runner, each
(variant x arch) push target runs on its own fresh runner with
outputs=type=image,push-by-digest=true,push=true (no local image
store), then a tiny merge job assembles the multi-arch manifest with
docker buildx imagetools create from digest artifacts. Per-runner disk
peak is roughly one-quarter of the old single-job peak. The four
Docker Hub tags per release are unchanged. As a bonus, amd64 and arm64
now build in parallel.

No image-level changes beyond the opencode bump.

Dockerfile

@@ -5,7 +5,7 @@ ARG DEBIAN_VERSION=trixie-slim
 FROM debian:${DEBIAN_VERSION} AS base
 
 ARG TARGETARCH
-ARG OPENCODE_VERSION=1.14.30
+ARG OPENCODE_VERSION=1.14.31
 
 LABEL maintainer="joakimp"
 LABEL description="Portable opencode developer container"