fix(ci): use native arm64 runners for multi-platform Docker builds

[Jomik]

Summary

Replace QEMU-emulated arm64 builds with native ubuntu-24.04-arm runners using the digest-merge pattern. The single push-image job is split into a build job (per-platform on native runners, push by digest) and a merge job (assemble multi-arch manifest via docker buildx imagetools create). This eliminates the ~40 minute QEMU bottleneck caused by compiling native modules (isolated-vm) under emulation.

Also changes the release schedule from daily to weekly (Mondays at 5am UTC) to reduce noise from Renovate patch bumps.

Decisions & callouts

• Cache is now scoped per matrix cell (scope=build-node{N}-linux-{arch}) to prevent cross-platform cache thrashing. The first run after merge will be a full cache miss.
• fail-fast: false on the build matrix so a failure on one platform doesn't cancel the other.
• All shell steps use set -euo pipefail for robustness.
• IMAGE_NAME is lowercased at job start because github.repository returns mixed-case which Docker rejects in the outputs name parameter.

Manual testing

• Trigger the workflow via workflow_dispatch on this branch
• Verify 4 build jobs run on correct runners (amd64 on ubuntu-latest, arm64 on ubuntu-24.04-arm)
• Verify 2 merge jobs complete and create multi-arch manifests
• Run docker buildx imagetools inspect ghcr.io/jomik/screeps-server:sha-09dbb3a and confirm both linux/amd64 and linux/arm64 are listed