chore: use CHANGELOG.md content for GitHub release notes (#420)

Replace auto-generated release notes with content extracted from
CHANGELOG.md for the specific version being released. This provides more
structured and consistent release notes with proper sections like Bug
Fixes, New Features, etc.

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

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

.github/workflows/create-release-tag.yml

@@ -24,12 +24,6 @@ jobs:
         VERSION="${BRANCH_NAME#release/v}"
         echo "version=$VERSION" >> $GITHUB_OUTPUT
     
-    - name: Get previous release tag
-      id: previous_tag
-      run: |
-        PREVIOUS_TAG=$(git describe --tags --abbrev=0 HEAD~1 2>/dev/null || echo "")
-        echo "previous_tag=$PREVIOUS_TAG" >> $GITHUB_OUTPUT
-
     - name: Create and push tag
       run: |
         git config --local user.email "github-actions[bot]@users.noreply.github.com"
@@ -46,14 +40,34 @@ jobs:
       env:
         GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |
-        # Create release with auto-generated notes
-        gh release create "v${{ steps.extract_version.outputs.version }}" \
-          --title "Release v${{ steps.extract_version.outputs.version }}" \
-          --generate-notes \
-          --notes-start-tag "${{ steps.previous_tag.outputs.previous_tag }}" \
-          --notes "Published to PyPI: https://pypi.org/project/claude-agent-sdk/${{ steps.extract_version.outputs.version }}/
+        VERSION="${{ steps.extract_version.outputs.version }}"
 
-        ### Installation
-        \`\`\`bash
-        pip install claude-agent-sdk==${{ steps.extract_version.outputs.version }}
-        \`\`\`"
+        # Extract changelog section for this version to a temp file
+        awk -v ver="$VERSION" '
+          /^## / {
+            if (found) exit
+            if ($2 == ver) found=1
+            next
+          }
+          found { print }
+        ' CHANGELOG.md > release_notes.md
+
+        # Append install instructions
+        cat >> release_notes.md << 'EOF'
+
+---
+
+**PyPI:** https://pypi.org/project/claude-agent-sdk/VERSION/
+
+```bash
+pip install claude-agent-sdk==VERSION
+```
+EOF
+
+        # Replace VERSION placeholder
+        sed -i "s/VERSION/$VERSION/g" release_notes.md
+
+        # Create release with notes from file
+        gh release create "v$VERSION" \
+          --title "v$VERSION" \
+          --notes-file release_notes.md