Add omnibus GitHub Release workflow

Single workflow triggered by v* tags that creates one unified
"Basecamp SDK vX.Y.Z" release with a combined changelog and
installation instructions for all five SDKs.

Uses AI inference for changelog summarization with a fallback to
raw commit list. Independent of the per-SDK publish workflows —
the release announcement doesn't gate artifact availability.

Author: jeremy

.github/workflows/release-github.yml

@@ -0,0 +1,160 @@
+name: Create GitHub Release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+permissions:
+  contents: write
+  models: read
+
+jobs:
+  release:
+    name: Create Release
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0
+
+      - name: Verify tag is on main
+        run: |
+          git fetch origin main
+          git merge-base --is-ancestor "$GITHUB_SHA" origin/main
+
+      - name: Extract version
+        id: version
+        run: |
+          VERSION="${GITHUB_REF#refs/tags/}"
+          SEMVER="${VERSION#v}"
+          echo "version=$VERSION" >> $GITHUB_OUTPUT
+          echo "semver=$SEMVER" >> $GITHUB_OUTPUT
+
+      - name: Build changelog prompt
+        id: changelog-inputs
+        run: |
+          PREV_TAG=$(git describe --tags --match 'v*' --abbrev=0 HEAD^ 2>/dev/null || true)
+          if [ -z "$PREV_TAG" ]; then
+            COMMITS=$(git log --pretty=format:"- %s (%h)")
+            git diff --stat HEAD > /tmp/sdk-diff.txt
+          else
+            COMMITS=$(git log --pretty=format:"- %s (%h)" "${PREV_TAG}..HEAD")
+            git diff --stat "${PREV_TAG}..HEAD" > /tmp/sdk-diff.txt
+          fi
+          echo "$COMMITS" > /tmp/commits.txt
+
+          {
+            echo "SDK: All (Go, TypeScript, Ruby, Swift, Kotlin)"
+            echo "Version: ${{ steps.version.outputs.version }}"
+            echo ""
+            echo "Commits since last release:"
+            cat /tmp/commits.txt
+            echo ""
+            echo "Diff (truncated):"
+            head -c 100000 /tmp/sdk-diff.txt
+          } > /tmp/user-message.txt
+
+          cp .github/prompts/summarize-changelog.prompt.yml /tmp/prompt.yml
+          python3 -c "
+          import yaml
+          with open('/tmp/prompt.yml') as f:
+              prompt = yaml.safe_load(f)
+          with open('/tmp/user-message.txt') as f:
+              user_msg = f.read()
+          prompt['messages'].append({'role': 'user', 'content': user_msg})
+          with open('/tmp/prompt.yml', 'w') as f:
+              yaml.dump(prompt, f, default_flow_style=False, allow_unicode=True)
+          "
+
+      - name: Summarize changes
+        id: summarize
+        continue-on-error: true
+        uses: actions/ai-inference@v1
+        with:
+          prompt-file: /tmp/prompt.yml
+
+      - name: Write changelog
+        if: steps.summarize.outcome == 'success'
+        run: |
+          VERSION="${{ steps.version.outputs.version }}"
+          SEMVER="${{ steps.version.outputs.semver }}"
+          RESPONSE_FILE="${{ steps.summarize.outputs.response-file }}"
+          {
+            cat "$RESPONSE_FILE"
+            echo ""
+            echo "## Installation"
+            echo ""
+            echo "**Go**"
+            echo '```'
+            echo "go get github.com/basecamp/basecamp-sdk/go@${VERSION}"
+            echo '```'
+            echo ""
+            echo "**TypeScript**"
+            echo '```'
+            echo "npm install @37signals/basecamp@${SEMVER}"
+            echo '```'
+            echo ""
+            echo "**Ruby**"
+            echo '```'
+            echo "gem install basecamp-sdk -v ${SEMVER}"
+            echo '```'
+            echo ""
+            echo "**Swift**"
+            echo '```swift'
+            echo ".package(url: \"https://github.com/basecamp/basecamp-sdk.git\", from: \"${SEMVER}\")"
+            echo '```'
+            echo ""
+            echo "**Kotlin**"
+            echo '```kotlin'
+            echo "implementation(\"com.basecamp:basecamp-sdk:${SEMVER}\")"
+            echo '```'
+          } > changelog.md
+
+      - name: Fallback changelog
+        if: steps.summarize.outcome != 'success'
+        run: |
+          VERSION="${{ steps.version.outputs.version }}"
+          SEMVER="${{ steps.version.outputs.semver }}"
+          {
+            echo "## Changes"
+            echo ""
+            cat /tmp/commits.txt
+            echo ""
+            echo "## Installation"
+            echo ""
+            echo "**Go**"
+            echo '```'
+            echo "go get github.com/basecamp/basecamp-sdk/go@${VERSION}"
+            echo '```'
+            echo ""
+            echo "**TypeScript**"
+            echo '```'
+            echo "npm install @37signals/basecamp@${SEMVER}"
+            echo '```'
+            echo ""
+            echo "**Ruby**"
+            echo '```'
+            echo "gem install basecamp-sdk -v ${SEMVER}"
+            echo '```'
+            echo ""
+            echo "**Swift**"
+            echo '```swift'
+            echo ".package(url: \"https://github.com/basecamp/basecamp-sdk.git\", from: \"${SEMVER}\")"
+            echo '```'
+            echo ""
+            echo "**Kotlin**"
+            echo '```kotlin'
+            echo "implementation(\"com.basecamp:basecamp-sdk:${SEMVER}\")"
+            echo '```'
+          } > changelog.md
+
+      - name: Create GitHub Release
+        uses: softprops/action-gh-release@v2
+        with:
+          name: Basecamp SDK ${{ steps.version.outputs.version }}
+          body_path: changelog.md
+          draft: false
+          prerelease: ${{ contains(steps.version.outputs.version, '-') }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}