chore: optimize devcontainer for Codespaces prebuilds

Move install commands from postCreateCommand to onCreateCommand so they
are cached during prebuild, reducing codespace startup time.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: DanWahlin

.devcontainer/devcontainer.json

@@ -12,6 +12,6 @@
       ]
     }
   },
-  "postCreateCommand": "pip install pytest && npm install -g @github/copilot && gh extension install github/gh-copilot",
+  "onCreateCommand": "pip install pytest && npm install -g @github/copilot && gh extension install github/gh-copilot",
   "postStartCommand": "echo '✅ Python, pytest, GitHub CLI, and Copilot CLI are ready. Run: cd samples/book-app-project && python book_app.py help'"
 }