Add PyPI publish workflow with duplicate version check (#129)

Backports the publish workflow from ReforgeHQ/sdk-python and adapts it
for prefab-cloud-python.

Key features:
- Triggers on push to main or manual workflow dispatch
- Checks PyPI before publishing to prevent duplicate releases
- Uses trusted publishing (OIDC) for secure authentication
- Runs tests before publishing
- Only publishes if version doesn't already exist on PyPI

Changes from sdk-python workflow:
- Uses pyproject.toml for version (instead of VERSION file)
- Uses prefab-cloud-python as PyPI package name
- Uses PREFAB_INTEGRATION_TEST_API_KEY secret
- Runs pytest with '-k not integration' filter (matches CI)

PyPI coordinates: https://pypi.org/project/prefab-cloud-python

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: jkebinger

.github/workflows/publish.yml

@@ -0,0 +1,71 @@
+name: "Publish to PyPI"
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    if: github.ref == 'refs/heads/main'
+    environment: release
+    permissions:
+      id-token: write # IMPORTANT: this permission is mandatory for trusted publishing
+      contents: read
+
+    steps:
+      - name: Checkout repo + submodules
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.10"
+
+      - name: Check if version already published
+        id: version-check
+        run: |
+          VERSION=$(grep '^version = ' pyproject.toml | sed 's/version = "\(.*\)"/\1/')
+          echo "Current version: $VERSION"
+
+          # Check if this version exists on PyPI
+          HTTP_STATUS=$(curl -s -o /dev/null -w "%{http_code}" "https://pypi.org/pypi/prefab-cloud-python/$VERSION/json")
+
+          if [ "$HTTP_STATUS" = "200" ]; then
+            echo "Version $VERSION already exists on PyPI"
+            echo "skip=true" >> $GITHUB_OUTPUT
+          else
+            echo "Version $VERSION not found on PyPI, proceeding with publish"
+            echo "skip=false" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Install Poetry
+        if: steps.version-check.outputs.skip == 'false'
+        uses: snok/install-poetry@v1
+        with:
+          virtualenvs-create: true
+
+      - name: Install dependencies
+        if: steps.version-check.outputs.skip == 'false'
+        run: |
+          poetry install --no-interaction
+
+      - name: Run tests
+        if: steps.version-check.outputs.skip == 'false'
+        run: poetry run pytest -k 'not integration'
+        env:
+          PREFAB_INTEGRATION_TEST_API_KEY: ${{ secrets.PREFAB_INTEGRATION_TEST_API_KEY }}
+
+      - name: Build package
+        if: steps.version-check.outputs.skip == 'false'
+        run: poetry build
+
+      - name: Publish to PyPI
+        if: steps.version-check.outputs.skip == 'false'
+        uses: pypa/gh-action-pypi-publish@release/v1
+        with:
+          packages-dir: dist/