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.

clipboard/darwin.go

@@ -0,0 +1,43 @@
+//go:build darwin
+
+package clipboard
+
+/*
+#cgo CFLAGS: -x objective-c
+#cgo LDFLAGS: -framework Cocoa
+#include <stdlib.h>
+#import <Cocoa/Cocoa.h>
+
+int copyFileToPasteboard(char* path) {
+    @autoreleasepool {
+        NSString *strPath = [NSString stringWithUTF8String:path];
+        if (!strPath) return 0;
+
+        NSPasteboard *pb = [NSPasteboard generalPasteboard];
+        [pb clearContents];
+        [pb declareTypes:@[NSFilenamesPboardType] owner:nil];
+        return [pb setPropertyList:@[strPath] forType:NSFilenamesPboardType] ? 1 : 0;
+    }
+}
+*/
+import "C"
+import (
+	"errors"
+	"path/filepath"
+	"unsafe"
+)
+
+func CopyFile(path string) error {
+	absPath, err := filepath.Abs(path)
+	if err != nil {
+		return err
+	}
+
+	cPath := C.CString(absPath)
+	defer C.free(unsafe.Pointer(cPath))
+
+	if success := C.copyFileToPasteboard(cPath); success == 0 {
+		return errors.New("failed to write to macOS pasteboard")
+	}
+	return nil
+}