fix: Install poetry via pipx (Python feature provides pipx, not poetry)

The Python devcontainer feature installs pipx but not poetry by default.
Updated to:
1. Add poetry to toolsToInstall in devcontainer.json
2. Use pipx to install poetry in oncreate.sh
3. Ensure PATH includes /usr/local/py-utils/bin where pipx installs tools

.devcontainer/platform/devcontainer.json

@@ -20,7 +20,8 @@
     },
     "ghcr.io/devcontainers/features/python:1": {
       "version": "3.13",
-      "installTools": true
+      "installTools": true,
+      "toolsToInstall": "flake8,autopep8,black,mypy,pytest,poetry"
     }
   },