Switch from swift-format to swift format in CI

Use the built-in swift format subcommand from the Swift toolchain instead
of cloning and building swift-format from source.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: RISCfuture

.github/workflows/ci.yml

@@ -48,12 +48,5 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Install swift-format
-        run: |
-          git clone --branch 600.0.0 --depth 1 https://github.com/swiftlang/swift-format.git /tmp/swift-format
-          cd /tmp/swift-format
-          swift build -c release
-          cp /tmp/swift-format/.build/release/swift-format /usr/local/bin/
-
       - name: Run swift-format
-        run: swift-format lint -r .
+        run: swift format lint -r .