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

[Copilot]

The repo ships a no-devcontainer coaching rule but has no devcontainer of its own. This adds one so contributors (and Codespaces users) get a reproducible environment matching CI.

Changes

• .devcontainer/devcontainer.json — Microsoft typescript-node:1-22-bookworm image (Node 22 LTS, matching .github/workflows/ci.yml), npm install as postCreateCommand, and the github-cli feature.
• Preinstalled VS Code extensions — ESLint, TypeScript Next, Vitest Explorer, Code Spell Checker, Playwright Test (covers every tool invoked by npm run check).
• Editor settings — format-on-save, ESLint fix-all on save, workspace TypeScript SDK.
• .devcontainer/README.md — brief usage notes for Codespaces and the Dev Containers extension.

No application code is touched.

[aymenfurter]

@copilot Lets go with mcr.microsoft.com/devcontainers/typescript-node:22-bookworm

[Copilot]

@copilot Lets go with mcr.microsoft.com/devcontainers/typescript-node:22-bookworm

Updated in 8ae4b8b — image is now mcr.microsoft.com/devcontainers/typescript-node:22-bookworm.

[github-actions]

⚠️ Deprecation Warning: The deny-licenses option is deprecated for possible removal in the next major release. For more information, see issue 997.

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None