fix(bootstrapper): keep the deprecation notices out of silent output

In transparent magic mode the bootstrapper is the paket.exe that
Paket.Restore.targets invokes with '-s' and whose stdout it parses as
MSBuild conditions; WriteAlways bypassed that '-s'.

Author: fpellet

RELEASE_NOTES.md

@@ -1,5 +1,5 @@
 #### 11.0.0-alpha001 - 2026-09-12
-* DEPRECATION: `paket.exe` and `paket.bootstrapper.exe` are deprecated and will no longer be published from Paket 12.0 onwards. The `paket` .NET tool becomes the only supported way to run Paket: `dotnet tool install paket`. Both binaries now print a notice on every run.
+* DEPRECATION: `paket.exe` and `paket.bootstrapper.exe` are deprecated and will no longer be published from Paket 12.0 onwards. The `paket` .NET tool becomes the only supported way to run Paket: `dotnet tool install paket`. Both binaries now print a notice on every run unless `-s` is passed.
 * The bootstrapper shipped with this release never resolves a version past 11.x, so it keeps working once 12.0 ships without a `paket.exe` asset instead of failing the build. An explicitly pinned version is unaffected.
 * Upgrade to .NET 10: SDK 10.0.400 and target frameworks from net9 to net10.0. The `paket` .NET tool now requires the .NET 10 runtime; the merged `paket.exe` still targets net461.
 * Allow a whole `source` line in paket.dependencies to come from an environment variable (`source %FEED_URL%`) - https://github.com/fsprojects/Paket/pull/4374

src/Paket.Bootstrapper/BootstrapperHelper.cs

@@ -11,7 +11,7 @@ namespace Paket.Bootstrapper
     internal static class BootstrapperHelper
     {
         /// <summary>
-        /// Printed on every run, whatever the verbosity. Paket 12.0 and later ship the .NET tool
+        /// Printed on every run that isn't silent. Paket 12.0 and later ship the .NET tool
         /// only, and their releases carry no paket.exe asset for the bootstrapper to download.
         /// </summary>
         public const string DeprecationNotice =

src/Paket.Bootstrapper/DownloadStrategies/DownloadStrategy.cs

@@ -52,7 +52,7 @@ internal static string CapToLastSupportedVersion(string version)
             if (parsed.Major <= LastSupportedMajorVersion)
                 return version;
 
-            ConsoleImpl.WriteAlways(
+            ConsoleImpl.WriteWarning(
                 "Paket {0} is available, but it is published as a .NET tool only and cannot be downloaded by the bootstrapper. Staying on {1}. To move on, run: dotnet tool install paket",
                 version, LastSupportedVersion);
 

src/Paket.Bootstrapper/Program.cs

@@ -61,10 +61,10 @@ static void Main(string[] args)
 
             ConsoleImpl.Verbosity = options.Verbosity;
 
-            // WriteAlways, not WriteWarning: the latter is dropped by the verbosity filter as soon
-            // as '-s' is passed, and transparent magic mode adds a '-s' of its own, which is exactly
-            // the population that has to see this.
-            ConsoleImpl.WriteAlways(BootstrapperHelper.DeprecationNotice);
+            // WriteWarning, not WriteAlways: it goes to stdout, and under '-s' (which transparent
+            // magic mode adds on its own) that stream is parsed by Paket.Restore.targets
+            // ('show-conditions -s'), where any extra line breaks the build.
+            ConsoleImpl.WriteWarning(BootstrapperHelper.DeprecationNotice);
 
             if (options.UnprocessedCommandArgs.Any())
                 ConsoleImpl.WriteWarning("Ignoring the following unknown argument(s): {0}", String.Join(", ", options.UnprocessedCommandArgs));