feat: reject cross-host redirects and HTTPS→HTTP downgrades (#95)

Add defaultCheckRedirect to both GitHub and Gitea clients that rejects:
- HTTPS→HTTP protocol downgrades (prevents plaintext leakage)
- Cross-host redirects entirely (prevents consuming untrusted responses)

Same-host, same-or-upgraded-scheme redirects remain allowed.

Both NewClient constructors wire the policy, and SetHTTPClient(nil)
restores it. Callers providing a non-nil client are responsible for
configuring their own safe redirect policy.

Closes #95

github/client_test.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
+	"strings"
 	"testing"
 	"time"
 )
@@ -407,3 +409,105 @@ func TestAPIError_Error_NewlineSanitized(t *testing.T) {
 		}
 	}
 }
+
+func TestNewClient_HasCheckRedirect(t *testing.T) {
+	c := NewClient("secret-token", "https://api.github.com")
+	if c.httpClient.CheckRedirect == nil {
+		t.Fatal("expected CheckRedirect to be set")
+	}
+}
+
+func TestDefaultCheckRedirect_RejectsHTTPSToHTTP(t *testing.T) {
+	prev := &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/foo"}}
+	req := &http.Request{
+		URL:    &url.URL{Scheme: "http", Host: "api.github.com", Path: "/foo"},
+		Header: http.Header{"Authorization": []string{"Bearer token"}},
+	}
+	err := defaultCheckRedirect(req, []*http.Request{prev})
+	if err == nil {
+		t.Fatal("expected error on HTTPS→HTTP redirect")
+	}
+	if !strings.Contains(err.Error(), "HTTPS to HTTP downgrade") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestDefaultCheckRedirect_RejectsCrossHost(t *testing.T) {
+	prev := &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/foo"}}
+	req := &http.Request{
+		URL:    &url.URL{Scheme: "https", Host: "objects.githubusercontent.com", Path: "/bar"},
+		Header: http.Header{"Authorization": []string{"Bearer token"}},
+	}
+	err := defaultCheckRedirect(req, []*http.Request{prev})
+	if err == nil {
+		t.Fatal("expected error on cross-host redirect")
+	}
+	if !strings.Contains(err.Error(), "cross-host") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestDefaultCheckRedirect_AllowsSameHost(t *testing.T) {
+	prev := &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/foo"}}
+	req := &http.Request{
+		URL:    &url.URL{Scheme: "https", Host: "api.github.com", Path: "/bar"},
+		Header: http.Header{"Authorization": []string{"Bearer token"}},
+	}
+	err := defaultCheckRedirect(req, []*http.Request{prev})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	// Auth should be preserved on same-host redirect
+	if auth := req.Header.Get("Authorization"); auth != "Bearer token" {
+		t.Errorf("expected Authorization to be preserved, got %q", auth)
+	}
+}
+
+func TestDefaultCheckRedirect_AllowsSameHostHTTPToHTTP(t *testing.T) {
+	prev := &http.Request{URL: &url.URL{Scheme: "http", Host: "localhost:8080", Path: "/foo"}}
+	req := &http.Request{
+		URL:    &url.URL{Scheme: "http", Host: "localhost:8080", Path: "/bar"},
+		Header: http.Header{},
+	}
+	err := defaultCheckRedirect(req, []*http.Request{prev})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestDefaultCheckRedirect_RejectsTooManyRedirects(t *testing.T) {
+	via := make([]*http.Request, 10)
+	for i := range via {
+		via[i] = &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/"}}
+	}
+	req := &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/final"}}
+	err := defaultCheckRedirect(req, via)
+	if err == nil {
+		t.Fatal("expected error after 10 redirects")
+	}
+	if !strings.Contains(err.Error(), "10 redirects") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestDefaultCheckRedirect_EmptyViaAllowed(t *testing.T) {
+	req := &http.Request{URL: &url.URL{Scheme: "https", Host: "api.github.com", Path: "/foo"}}
+	err := defaultCheckRedirect(req, nil)
+	if err != nil {
+		t.Fatalf("unexpected error with empty via: %v", err)
+	}
+}
+
+func TestSetHTTPClient_NilRestoresDefault(t *testing.T) {
+	c := NewClient("token", "https://api.github.com")
+	c.SetHTTPClient(nil)
+	if c.httpClient == nil {
+		t.Fatal("expected non-nil httpClient after SetHTTPClient(nil)")
+	}
+	if c.httpClient.Timeout != 30*time.Second {
+		t.Errorf("expected 30s timeout, got %v", c.httpClient.Timeout)
+	}
+	if c.httpClient.CheckRedirect == nil {
+		t.Fatal("expected CheckRedirect policy after SetHTTPClient(nil)")
+	}
+}