feat: disable backup feature by default (#511)

* feat: disable backup feature by default

Backup is now opt-in rather than opt-out. Users who need backup
before applying updates must explicitly set BackupEnabled = true
or set the BACKUP environment variable to true.

This reduces unnecessary I/O and storage consumption for users who
don't require rollback capability. Rollback via backup remains fully
functional when enabled.

Option.BackupEnabled default: true -> false
BackupConfig.Enabled default:     true -> false

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

* fix: update backup tests to match new default (false)

Test assertions that expected BackupEnabled default to be true now
expect false. Also renamed Options_BackupEnabled_DefaultIsTrue to
Options_BackupEnabled_DefaultIsFalse for clarity.

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

---------

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

Author: JusterZhu

src/c#/GeneralUpdate.Core/Configuration/Option.cs

@@ -124,7 +124,7 @@ public override bool Equals(object? obj)
 
         public static Option<bool?> PatchEnabled { get; } = ValueOf<bool?>("PATCH", true);
 
-        public static Option<bool?> BackupEnabled { get; } = ValueOf<bool?>("BACKUP", true);
+        public static Option<bool?> BackupEnabled { get; } = ValueOf<bool?>("BACKUP", false);
 
         public static Option<bool> Silent { get; } = ValueOf<bool>("ENABLESILENTUPDATE", false);
 

src/c#/GeneralUpdate.Core/Configuration/UpdateContext.cs

@@ -120,7 +120,7 @@ public class UpdateContext : UpdateConfiguration
 
     /// <summary>
     ///     Whether to back up the current version before applying an update.
-    ///     Computed from <see cref="Option.BackupEnabled" />, defaults to <c>true</c>.
+    ///     Computed from <see cref="Option.BackupEnabled" />, defaults to <c>false</c>.
     /// </summary>
     public bool? BackupEnabled { get; set; }
 

src/c#/GeneralUpdate.Core/FileSystem/StorageManager.cs

@@ -727,9 +727,9 @@ public sealed class BackupConfig
         public List<string> Directories { get; set; } = new();
 
         /// <summary>
-        /// Whether the backup feature is enabled. Default is <c>true</c>.
+        /// Whether the backup feature is enabled. Default is <c>false</c>.
         /// </summary>
-        public bool Enabled { get; set; } = true;
+        public bool Enabled { get; set; } = false;
     }
 
     /// <summary>

tests/CoreTest/Configuration/UpdateContextWiringTests.cs

@@ -205,8 +205,8 @@ public void Options_VerifyChecksum_DefaultIsTrue()
         => Assert.True(Option.VerifyChecksum.DefaultValue);
 
     [Fact]
-    public void Options_BackupEnabled_DefaultIsTrue()
-        => Assert.True(Option.BackupEnabled.DefaultValue);
+    public void Options_BackupEnabled_DefaultIsFalse()
+        => Assert.False(Option.BackupEnabled.DefaultValue);
 
     [Fact]
     public void Options_PatchEnabled_DefaultIsTrue()

tests/CoreTest/Configuration/UpdateOptionsStaticTests.cs

@@ -49,7 +49,7 @@ public void PatchEnabled_HasCorrectDefault()
     [Fact]
     public void BackupEnabled_HasCorrectDefault()
     {
-        Assert.True(Option.BackupEnabled.DefaultValue);
+        Assert.False(Option.BackupEnabled.DefaultValue);
     }
 
     [Fact]