feat: register Discord session handlers and permission checker in DI

Task 5: DI wiring for DiscordNewSessionHandler, DiscordListSessionsHandler,
DiscordPermissionChecker, and DiscordPlatformMessenger.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.DiscordBot/Program.cs

@@ -1,5 +1,8 @@
 using GmRelay.DiscordBot;
+using GmRelay.DiscordBot.Features.Sessions;
+using GmRelay.DiscordBot.Infrastructure.Discord;
 using GmRelay.DiscordBot.Infrastructure.Logging;
+using GmRelay.Shared.Platform;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using NetCord;
@@ -37,6 +40,11 @@ builder.Services.AddSingleton<NpgsqlDataSource>(sp =>
     return NpgsqlDataSource.Create(connectionString);
 });
 
+builder.Services.AddSingleton<DiscordPermissionChecker>();
+builder.Services.AddSingleton<DiscordListSessionsHandler>();
+builder.Services.AddSingleton<DiscordNewSessionHandler>();
+builder.Services.AddSingleton<IPlatformMessenger, DiscordPlatformMessenger>();
+
 builder.Services
     .AddDiscordGateway(options =>
     {

tests/GmRelay.Bot.Tests/Discord/DiscordStartupTests.cs

@@ -71,6 +71,17 @@ public sealed class DiscordStartupTests
         Assert.DoesNotContain("Token", logger);
     }
 
+    [Fact]
+    public void Program_ShouldRegisterDiscordSessionHandlers()
+    {
+        var program = ReadProgram();
+        Assert.Contains("DiscordListSessionsHandler", program);
+        Assert.Contains("DiscordNewSessionHandler", program);
+        Assert.Contains("DiscordPermissionChecker", program);
+        Assert.Contains("DiscordPlatformMessenger", program);
+        Assert.Contains("IPlatformMessenger", program);
+    }
+
     private static string ReadProgram()
     {
         var repoRoot = GetRepoRoot();