Fix release workflow: cross-compile darwin/amd64 on arm64 runner

macos-13 runners are retired. Use macos-latest (arm64) for both darwin
builds, with clang -arch x86_64 to cross-compile the amd64 variant.

https://claude.ai/code/session_01688MBwKW1GJtJKoiYvMxke

Author: claude

.github/workflows/release.yml

@@ -22,7 +22,7 @@ jobs:
           - os: macos-latest
             goos: darwin
             goarch: arm64
-          - os: macos-13
+          - os: macos-latest
             goos: darwin
             goarch: amd64
 
@@ -43,6 +43,7 @@ jobs:
           GOOS: ${{ matrix.goos }}
           GOARCH: ${{ matrix.goarch }}
           CGO_ENABLED: '1'
+          CC: ${{ matrix.goarch == 'amd64' && matrix.goos == 'darwin' && 'clang -arch x86_64' || '' }}
         run: go build -ldflags="-s -w" -o microchat-${{ matrix.goos }}-${{ matrix.goarch }} ./cmd/server/
 
       - uses: actions/upload-artifact@v4