Per-language devcontainers for demos

[zeevdr]

Description

Different demos use Go / Python / TypeScript. Each demo subfolder should have its own language-tuned devcontainer (slim base, preinstalled SDK). Reduces image size vs. monolithic container.

Acceptance criteria

• Per-demo .devcontainer/ (Go, Python, TS subfolders)
• Each uses a slim language base image
• Preinstalls the relevant decree SDK
• CI matrix tests each demo boots and runs its main script
• README doc: how to pick a demo, what toolchain each uses