fix: harden non-nix scroll prebuilds

[MarcStdt]

Summary

• replace the removed LGSM prebuild path with a Docker install flow from checked-out scroll source
• remove Nix/highcard image references from release and prebuild paths
• pin runtime metadata to artifacts.druid.gg/druid-team/druid:v0.1.243 and v0.1.243-steamcmd
• publish prebuild metadata through druid push, preserving ports/resources/category where intended
• support one target, comma-separated targets, or all-steam in the manual prebuild workflow

Validation

• druid-cli v0.1.243 release passed, including SteamCMD Docker build
• docker pull/probe artifacts.druid.gg/druid-team/druid:v0.1.243-steamcmd
• make build-tree
• ./scripts/validate_all_scrolls.sh
• rg no stale Nix/highcard/v0.1.241/v0.1.242/pre-refactor registry command refs in production paths
• git diff --check

Local Steam prebuild note

• On this local arm64 Docker host, SteamCMD's 32-bit i386 binary exits before output even in upstream gameservermanagers/steamcmd:ubuntu-24.04. The CS2 no-push prebuild reaches LGSM dependency checks cleanly, then cannot complete the SteamCMD app download locally because of that host/emulation limit.
• I will run the Prebuild Steam Scrolls workflow on the PR branch on the amd64 nauen runner for the actual Steam proof before merge.

[druid-infra]

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