client: actually write incoming messages to the OS clipboard

The MVP only printed to stdout. Now the listener calls
clipboard.WriteAll on every received message, except when the message
originated from itself (to avoid clobbering local edits with our own
prior send).

Adds:
- github.com/atotto/clipboard (cross-platform: Win/macOS/Linux)
- -no-clipboard flag for stdout-only mode
- "→ clipboard updated" trace line so the user can confirm the write

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

client/main.go

@@ -1,5 +1,5 @@
-// tether-client: connects to a tether-server, prints messages to stdout.
-// MVP — does not yet touch the OS clipboard.
+// tether-client: connects to a tether-server, writes incoming messages
+// to the OS clipboard and prints them to stdout.
 package main
 
 import (
@@ -14,6 +14,13 @@ import (
 	"os"
 	"strings"
 	"time"
+
+	"github.com/atotto/clipboard"
+)
+
+var (
+	noClipboard bool
+	myLabel     string
 )
 
 type Message struct {
@@ -26,13 +33,20 @@ func main() {
 	server := flag.String("server", "http://localhost:8765", "tether-server base URL")
 	label := flag.String("label", "linux-client", "X-Tether-Client label")
 	sendText := flag.String("send", "", "send this text and exit (otherwise listen)")
+	flag.BoolVar(&noClipboard, "no-clipboard", false, "don't write incoming messages to the OS clipboard")
 	flag.Parse()
+	myLabel = *label
 
 	if *sendText != "" {
 		send(*server, *label, *sendText)
 		return
 	}
 
+	if !noClipboard && clipboard.Unsupported {
+		fmt.Fprintln(os.Stderr, "tether-client: OS clipboard unsupported on this platform; falling back to stdout-only")
+		noClipboard = true
+	}
+
 	for {
 		if err := listen(*server, *label); err != nil {
 			log.Printf("stream error: %v — reconnecting in 3s", err)
@@ -85,6 +99,14 @@ func listen(server, label string) error {
 				if err := json.Unmarshal([]byte(data), &m); err == nil {
 					ts := time.UnixMilli(m.TS).Format("15:04:05")
 					fmt.Printf("\n──────  %s  from %s  ──────\n%s\n", ts, m.Source, m.Text)
+					// Don't echo our own message back into our own clipboard
+					if !noClipboard && m.Source != myLabel {
+						if err := clipboard.WriteAll(m.Text); err != nil {
+							fmt.Fprintf(os.Stderr, "  ! clipboard write error: %v\n", err)
+						} else {
+							fmt.Fprintln(os.Stderr, "  → clipboard updated")
+						}
+					}
 				}
 			}
 			ev, data = "", ""