Fix deadlock when UDP port is already in use (#58)

* Fix deadlock when UDP port 7734 is already in use

When listenUDP failed silently and FSWatch was disabled, the Run() select
loop had no way to receive events — causing a deadlock panic on kill or
hanging forever. Now listenUDP signals startup success/failure via a
buffered channel; if the port is already bound (watch already running),
we return a clear error immediately instead of hanging.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Distinguish EADDRINUSE from other UDP bind errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: greynewell

internal/files/daemon.go

@@ -4,13 +4,15 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net"
 	"os"
 	"path/filepath"
 	"sort"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"github.com/supermodeltools/cli/internal/api"
@@ -84,7 +86,17 @@ func (d *Daemon) Run(ctx context.Context) error {
 
 	d.logf("[step:2] Starting listeners")
 	if d.cfg.NotifyPort > 0 {
-		go d.listenUDP(ctx)
+		udpReady := make(chan error, 1)
+		go d.listenUDP(ctx, udpReady)
+		if err := <-udpReady; err != nil {
+			if !d.cfg.FSWatch {
+				if errors.Is(err, syscall.EADDRINUSE) {
+					return fmt.Errorf("UDP port %d already in use — is `supermodel watch` already running?", d.cfg.NotifyPort)
+				}
+				return fmt.Errorf("failed to start UDP listener on port %d: %w", d.cfg.NotifyPort, err)
+			}
+			d.logf("Warning: UDP listener failed (FSWatch active, continuing): %v", err)
+		}
 	}
 
 	if d.cfg.FSWatch {
@@ -525,14 +537,15 @@ func (d *Daemon) computeAffectedFiles(changedFiles []string) []string {
 	return daemonSortedKeys(affected)
 }
 
-func (d *Daemon) listenUDP(ctx context.Context) {
+func (d *Daemon) listenUDP(ctx context.Context, ready chan<- error) {
 	addr := fmt.Sprintf("127.0.0.1:%d", d.cfg.NotifyPort)
 	conn, err := net.ListenPacket("udp", addr)
 	if err != nil {
-		d.logf("UDP listener failed: %v", err)
+		ready <- err
 		return
 	}
 	defer conn.Close()
+	ready <- nil
 	d.logf("UDP listener on %s", addr)
 
 	go func() {