feat: add devcontainer config and Codespaces badge

Add .devcontainer/devcontainer.json using the standard Python 3.12
devcontainer image, with VS Code extensions for Python, Ruff, and
mypy. postCreateCommand installs dev dependencies via pip.

Add an "Open in GitHub Codespaces" badge to README.md.

Closes #9

Author: zeevdr

.devcontainer/devcontainer.json

@@ -0,0 +1,14 @@
+{
+  "name": "OpenDecree Python SDK",
+  "image": "mcr.microsoft.com/devcontainers/python:3.12",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-python.python",
+        "charliermarsh.ruff",
+        "ms-python.mypy-type-checker"
+      ]
+    }
+  },
+  "postCreateCommand": "cd sdk && pip install -e '.[dev]'"
+}

README.md

@@ -7,6 +7,7 @@
 [![License](https://img.shields.io/github/license/opendecree/decree-python)](LICENSE)
 [![Project Status: WIP](https://www.repostatus.org/badges/latest/wip.svg)](https://www.repostatus.org/#wip)
 [![codecov](https://codecov.io/gh/opendecree/decree-python/graph/badge.svg)](https://codecov.io/gh/opendecree/decree-python)
+[![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/opendecree/decree-python)
 
 Python SDK for [OpenDecree](https://github.com/opendecree/decree) — schema-driven configuration management.