Add VS Code Dev Container (`.devcontainer`) support

[ramsafin]

I'd like to provide a fully-containerized development environment using Docker + VS Code’s Remote-Containers feature. That will let new contributors get up and running with zero local setup (no need to install CMake, Ninja, compilers, sanitizers, etc.) - just “Open Folder in Container” in VS Code.

1. Create a .devcontainer/ directory at the project root.
2. Add a Dockerfile that
   • Starts from a base image (e.g. ubuntu:24.04)
   • Installs all build dependencies: CMake, Ninja, gcc/g++/clang/ccache, git, curl, etc.
   • Sets up a non-root user (vscode or similar)
3. Add devcontainer.json with:
   • "build": { "dockerfile": "Dockerfile" }
   • Workspace mount (default)
   • Recommended extensions (see .vscode/extensions.json)
   • Forward any ports (if your CLI ever serves a HTTP preview)
   • "remoteUser": "vscode"
   • postCreateCommand: e.g. cmake --preset gcc-RelWithDebInfo
4. Update README.md with a “Getting Started in Docker” section describing:
   • How to install Docker + VS Code + Remote Containers
   • “Open Folder in Container”
   • How to configure presets inside the container
   • How to run build/tests from inside VS Code or via CLI