Optimize DevContainer CI build time

[fjmrytfjsn]

Summary

• consolidate CI into one devcontainer job to avoid duplicate builds
• enable GitHub Actions cache for devcontainer image build
• set common-utils upgradePackages to false for faster builds
• parallelize missing npm global installs in setup.sh

Why

• current workflow spends most time in repeated devcontainer build phases
• this change targets the largest bottlenecks while keeping behavior intact

Validation

• workflow syntax and file checks pass locally
• branch pushed and ready for CI timing comparison