feat(release): package binaries in archives

- Build binary as `core` (or `core.exe` on Windows)
- Package in tar.gz (unix) or zip (windows)
- Archive names: core-{os}-{arch}.tar.gz/.zip

This prepares for dogfooding: host-uk/build can download and extract
the core CLI to replace complex GitHub Actions with simple commands.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -52,9 +52,25 @@ jobs:
         run: |
           mkdir -p dist
           go build -ldflags="-s -w -X main.Version=${{ github.ref_name }}" \
-            -o dist/core-${{ matrix.goos }}-${{ matrix.goarch }}${{ matrix.ext }} \
+            -o dist/core${{ matrix.ext }} \
             .
 
+      - name: Create archive (Unix)
+        if: matrix.goos != 'windows'
+        shell: bash
+        run: |
+          cd dist
+          tar -czvf core-${{ matrix.goos }}-${{ matrix.goarch }}.tar.gz core
+          rm core
+
+      - name: Create archive (Windows)
+        if: matrix.goos == 'windows'
+        shell: bash
+        run: |
+          cd dist
+          zip core-${{ matrix.goos }}-${{ matrix.goarch }}.zip core.exe
+          rm core.exe
+
       - name: Upload artifact
         uses: actions/upload-artifact@v4
         with: