Add dev container for Codespaces support

[pamelafox]

What

Add a dev container so the repo opens cleanly in GitHub Codespaces / VS Code Dev Containers with az, azd, and Python preinstalled.

Why

Lets new users try the starter kit without installing the Azure CLI, Azure Developer CLI, or Python toolchain locally — one click "Open in Codespaces" and azd up works.

How

Adds devcontainer.json based on the mcr.microsoft.com/devcontainers/python:3.13-bookworm image with:

• azure-cli and azure-dev/azd dev container features
• VS Code extensions: ms-azuretools.azure-dev, ms-azuretools.vscode-bicep, ms-python.python

Verification

Built and opened the dev container; az, azd, and python3 are on PATH. No infra or runtime code changed.

• azd up succeeded end-to-end on a fresh env (Bicep)
• azd up succeeded end-to-end on a fresh env (Terraform)
• python src/hello_claude.py returned a live model response
• N/A — docs-only / CI-only change

Checklist

• Mirrored the change across both infra-bicep and infra-terraform (if applicable) — N/A, no IaC changes
• No secrets / subscription IDs / tenant IDs / object IDs in commits, comments, or PR body
• README updated if user-facing behavior changed — N/A
• Conventional one logical change per PR