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.

main.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path/filepath"
 	"time"
+
+	"github.com/skidoodle/ctx/clipboard"
 )
 
 const defaultOutputFilename = "ctx.txt"
@@ -21,6 +23,7 @@ func run() error {
 	flag.Usage = usage
 	configFlag := flag.Bool("config", false, "open global ignore file for editing")
 	outputFlag := flag.String("o", defaultOutputFilename, "output filename")
+
 	flag.Parse()
 
 	if *configFlag {
@@ -62,6 +65,12 @@ func run() error {
 		return fmt.Errorf("writing output: %w", err)
 	}
 
+	if err := clipboard.CopyFile(*outputFlag); err != nil {
+		fmt.Fprintf(os.Stderr, "ctx: warning: clipboard copy failed: %v\n", err)
+	} else {
+		fmt.Printf("ctx: copied %s to clipboard\n", *outputFlag)
+	}
+
 	duration := time.Since(start).Round(time.Millisecond)
 	fmt.Printf("ctx: generated %s (%d files, ~%d tokens) in %v\n",
 		*outputFlag, len(files), tokenCount, duration)
@@ -75,7 +84,5 @@ func usage() {
 	fmt.Fprintf(os.Stderr, "  -config      Open global ignore file for editing\n")
 	fmt.Fprintf(os.Stderr, "  -o <file>    Output filename (default %q)\n", defaultOutputFilename)
 	fmt.Fprintf(os.Stderr, "\nExamples:\n")
-	fmt.Fprintf(os.Stderr, "  ctx .                  # Generate context for current dir\n")
-	fmt.Fprintf(os.Stderr, "  ctx -o out.txt src/    # Scan src/ and save to out.txt\n")
-	fmt.Fprintf(os.Stderr, "  ctx -config            # Open global ignore file\n")
+	fmt.Fprintf(os.Stderr, "  ctx .              # Generate and copy file\n")
 }