Package taskito-server as a multiarch distroless image

[pratyush618]

Add docker/scheduler.Dockerfile (distroless, static Rust binary, linux/amd64 + linux/arm64) and a publish workflow alongside the existing per-SDK ones.

The binary carries no language runtime, so one image serves Python, Node and Java apps. glibc and musl variants are unnecessary for a static build.

Version must come from the root Cargo.toml [workspace.package] and be covered by scripts/version.mjs so --check gates it in CI, matching the single-version rule.

Done when the image builds in CI for both architectures and --version matches version.mjs. Plan: tasks/executor-attach.md.

[pratyush618]

Shipped in #610 (squash 3b008960): docker/scheduler.Dockerfile builds the static binary on alpine musl onto distroless/static, publish-server.yml merges the two native-runner builds into a GHCR manifest list, ci-server-image.yml gates both architectures on PRs, and version.mjs --check guards a hardcoded ARG VERSION. Both image jobs passed on the merged run.