fix(#130): enforce HTTPS scheme consistently in GitHub client write methods

PostReview, DeleteReview, and RequestReviewer were calling c.httpClient.Do
directly, bypassing the scheme check in doRequest that rejects http:// URLs
unless AllowInsecureHTTP is explicitly enabled.

Introduce doRequestWithBody(ctx, method, url, body) with the same HTTPS guard,
and refactor all three write methods to use it. This ensures tokens are never
sent over plaintext regardless of which API path is exercised.

Add scheme validation tests for each method.

github/client_test.go

@@ -1077,6 +1077,42 @@ func TestRequestReviewer_Success(t *testing.T) {
 	}
 }
 
+func TestPostReview_RejectsHTTP(t *testing.T) {
+	// PostReview must reject http:// base URLs — tokens must not be sent in plaintext.
+	c := NewClient("tok", "http://127.0.0.1:1")
+	_, err := c.PostReview(context.Background(), "owner", "repo", 1, "APPROVE", "body", "", nil)
+	if err == nil {
+		t.Fatal("expected error for HTTP base URL in PostReview")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestDeleteReview_RejectsHTTP(t *testing.T) {
+	// DeleteReview must reject http:// base URLs — tokens must not be sent in plaintext.
+	c := NewClient("tok", "http://127.0.0.1:1")
+	err := c.DeleteReview(context.Background(), "owner", "repo", 1, 42)
+	if err == nil {
+		t.Fatal("expected error for HTTP base URL in DeleteReview")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
+func TestRequestReviewer_RejectsHTTP(t *testing.T) {
+	// RequestReviewer must reject http:// base URLs — tokens must not be sent in plaintext.
+	c := NewClient("tok", "http://127.0.0.1:1")
+	err := c.RequestReviewer(context.Background(), "owner", "repo", 1, "reviewer1")
+	if err == nil {
+		t.Fatal("expected error for HTTP base URL in RequestReviewer")
+	}
+	if !strings.Contains(err.Error(), "refusing HTTP request") {
+		t.Errorf("unexpected error message: %v", err)
+	}
+}
+
 func TestEscapePath_SpecialChars(t *testing.T) {
 	tests := []struct {
 		input string