fix: bind to correct host when probing for available ports

FindAvailablePort was checking :port (all interfaces) but the
server binds to host:port (127.0.0.1). If 127.0.0.1:8080 was
already in use, the probe could pass on 0.0.0.0:8080 and the
actual bind would fail. Now probes the same host:port the server
will use.

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

Author: wesm

cmd/agentsview/main.go

@@ -115,7 +115,7 @@ func runServe(args []string) {
 
 	go startPeriodicSync(engine)
 
-	port := server.FindAvailablePort(cfg.Port)
+	port := server.FindAvailablePort(cfg.Host, cfg.Port)
 	if port != cfg.Port {
 		fmt.Printf("Port %d in use, using %d\n", cfg.Port, port)
 	}

internal/server/server.go

@@ -7,6 +7,7 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	gosync "sync"
 	"time"
@@ -200,10 +201,12 @@ func (s *Server) Shutdown(ctx context.Context) error {
 	return srv.Shutdown(ctx)
 }
 
-// FindAvailablePort finds an available port starting from the given port.
-func FindAvailablePort(start int) int {
+// FindAvailablePort finds an available port starting from the
+// given port, binding to the specified host.
+func FindAvailablePort(host string, start int) int {
 	for port := start; port < start+100; port++ {
-		ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+		addr := net.JoinHostPort(host, strconv.Itoa(port))
+		ln, err := net.Listen("tcp", addr)
 		if err == nil {
 			ln.Close()
 			return port

internal/server/server_test.go

@@ -133,7 +133,7 @@ func (te *testEnv) writeSessionFile(
 // base URL. The server is shut down when the test finishes.
 func (te *testEnv) listenAndServe(t *testing.T) string {
 	t.Helper()
-	port := server.FindAvailablePort(40000)
+	port := server.FindAvailablePort("127.0.0.1", 40000)
 	te.srv.SetPort(port)
 
 	var serveErr error