Add Codespaces devcontainer

[Mirochill]

Summary

• Add a Codespaces/devcontainer setup for Python 3.12.
• Install uv from the official ghcr.io/astral-sh/uv image and use it to create .venv with sdk[dev] dependencies after container creation.
• Add an "Open in GitHub Codespaces" badge to the README.

Related issues

Closes #9

Test plan

• devcontainer.json parses as JSON
• git diff --check HEAD~1..HEAD
• make lint passes
• make typecheck passes
• make test passes

Not run locally: make lint, make typecheck, make test.

Checklist

• SDK regenerated if proto changed in decree (not applicable)
• Breaking change flagged in title/description if applicable (not applicable)
• Changelog/release note added if applicable (not applicable)

[github-actions]

Thanks for your first pull request! We appreciate the contribution.

Before review, please make sure tests pass: cd sdk && pytest

[zeevdr]

Thanks for this, Miro — devcontainer support for the Python SDK is genuinely useful and the venv/pytest/ruff wiring here is solid. 🙏

One change before merge: please align with the pattern the core decree repo already uses, so our devcontainers stay consistent across the org.

decree/.devcontainer doesn't use a custom Dockerfile. It uses official ghcr.io/devcontainers/features/* for the toolchain and a small setup script for anything not covered by a feature. Could you refactor 108 the same way:

1. Drop .devcontainer/Dockerfile.
2. Add the official Python feature instead:

   "features": {
     "ghcr.io/devcontainers/features/python:1": { "version": "3.12" }
   }

3. Install uv via the official astral installer in an onCreateCommand (pin preserved via the versioned URL):

   "onCreateCommand": "curl -LsSf https://astral.sh/uv/0.11.16/install.sh | sh"

Keep everything else as-is — the postCreateCommand venv install, the remoteEnv PATH, the VS Code settings/extensions, and the README badge are all good.

We avoid community/third-party devcontainer features by policy (vanilla-tooling only), which is why the official Python feature + astral's own installer is the preferred route here rather than a uv feature image.