Keep neoirc-cli sessions on localhost and test the relay goroutine stop

The auth cookie is always Secure, and Go 1.24's cookie jar sends a
Secure cookie only over HTTPS, so neoirc-cli lost its session against a
plain-HTTP server on localhost. Its jar now treats a server on localhost
as HTTPS, as curl does. The README's Transport Security section and
Python example say what a client needs.

A test checks that closing a registered IRC client's connection stops
its relay goroutine. The README Entrypoints section names a make target
only for the scripts that have one, and the Dockerfile says why the
test phase runs with -p 4.

Model: opus-5-5

internal/cli/api/client.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/http/cookiejar"
 	"net/url"
@@ -41,11 +42,34 @@ func NewClient(baseURL string) *Client {
 		BaseURL: baseURL,
 		HTTPClient: &http.Client{ //nolint:exhaustruct // defaults fine
 			Timeout: httpTimeout,
-			Jar:     jar,
+			Jar:     loopbackJar{CookieJar: jar},
 		},
 	}
 }
 
+// loopbackJar also sends the server's auth cookie, which is
+// always Secure, over plain HTTP to a server on localhost, as
+// curl does. Go 1.24's cookie jar sends a Secure cookie only
+// over HTTPS.
+type loopbackJar struct {
+	http.CookieJar
+}
+
+// Cookies returns the cookies to send to target, treating a
+// plain-HTTP target on localhost as HTTPS.
+func (jar loopbackJar) Cookies(target *url.URL) []*http.Cookie {
+	host := target.Hostname()
+	loopback := host == "localhost" || net.ParseIP(host).IsLoopback()
+
+	if target.Scheme == "http" && loopback {
+		secure := *target
+		secure.Scheme = "https"
+		target = &secure
+	}
+
+	return jar.CookieJar.Cookies(target)
+}
+
 // CreateSession creates a new session on the server.
 // If the server requires hashcash proof-of-work, it
 // automatically fetches the difficulty and computes a