Internalize Fallout.Cli types that aren't external contracts

The migrated commands are all `internal sealed`, but three types were
still `public` with no consumer outside the assembly and no PublicAPI
tracking: RunCommand (the lone public command), ProjectUpdater, and
Program (the tool entry point). None are part of a public surface —
there is no public command SDK yet (that's milestone #7).

Make them internal to match the rest of the command set. Fallout.Cli.Specs
already has InternalsVisibleTo, so the specs are unaffected; DI registration
works unchanged with internal types.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ChrisonSimtian

src/Fallout.Cli/Commands/RunCommand.cs

@@ -15,7 +15,7 @@ namespace Fallout.Cli.Commands;
 /// <c>fallout :run</c> (and the default, command-less invocation): builds the build project and
 /// runs it, forwarding any remaining arguments to the build.
 /// </summary>
-public sealed class RunCommand : IFalloutCommand
+internal sealed class RunCommand : IFalloutCommand
 {
     public string Name => "run";
 

src/Fallout.Cli/Program.cs

@@ -13,7 +13,7 @@
 
 namespace Fallout.Cli;
 
-public partial class Program
+internal partial class Program
 {
     private static async Task<int> Main(string[] args)
     {

src/Fallout.Cli/ProjectUpdater.cs

@@ -9,7 +9,7 @@
 
 namespace Fallout.Cli;
 
-public static class ProjectUpdater
+internal static class ProjectUpdater
 {
     public static async Task UpdateAsync(string projectFile)
     {