dx: add .devcontainer/ for one-click contributor setup

[boldsbrainai]

Summary

The extension ships a built-in rule (no-devcontainer.md) that coaches users to add a devcontainer to their own projects — yet the AI-Engineering-Coach repo itself has no .devcontainer/. This is an ironic gap and creates friction for new contributors.

Fix

Add .devcontainer/devcontainer.json (and optionally a Dockerfile) pre-configured with:

• Node.js LTS
• npm install post-create command
• Recommended VS Code extensions (ESLint, TypeScript, Vitest)

Effort estimate

~2 hours