Add cross-platform clipboard support

Implement CopyFile for darwin, linux, and windows and call it from
main to copy generated output to the clipboard. Enable CGO for builds,
update goreleaser and release workflow to use macOS, and add a
cross-platform test workflow that verifies the clipboard.

.github/workflows/test.yaml

@@ -0,0 +1,52 @@
+name: Test Build
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: stable
+
+      - name: Enable CGO (Mac/Linux only)
+        if: matrix.os != 'windows-latest'
+        run: echo "CGO_ENABLED=1" >> $GITHUB_ENV
+
+      - name: Build
+        run: go build -v .
+
+      - name: Verify Clipboard (Linux)
+        if: matrix.os == 'ubuntu-latest'
+        run: |
+          sudo apt-get update && sudo apt-get install -y xclip xvfb
+
+          xvfb-run sh -c '
+            ./ctx .
+            CLIP_CONTENT=$(xclip -selection clipboard -o -t text/uri-list)
+            echo "Clipboard contains: $CLIP_CONTENT"
+            echo "$CLIP_CONTENT" | grep "file://" || exit 1
+            echo "$CLIP_CONTENT" | grep "ctx.txt" || exit 1
+          '
+
+      - name: Verify Clipboard (Windows)
+        if: matrix.os == 'windows-latest'
+        run: |
+          ./ctx.exe .
+          $files = Get-Clipboard -Format FileDropList
+          if ($files) {
+            Write-Host "Clipboard contains: $files"
+            if ($files.Name -contains "ctx.txt") { exit 0 }
+          }
+          Write-Error "Clipboard did not contain ctx.txt"
+          exit 1