feat(discord): step-by-step game/pool creation wizard (issue #112)

Add Discord adapter for the platform-neutral wizard moved to Shared in
the previous commit. Six new files in src/GmRelay.DiscordBot/Features/
Sessions/Wizard/:

- DiscordWizardContextStore: IWizardContextStore abstraction +
  thread-safe in-memory impl keyed by draft id. Holds the (guild,
  channel, message) coordinates the messenger needs to re-send the
  draft after a 15-minute interaction token expires.
- DiscordWizardStep: renderer for all 15 wizard steps. Returns an
  embed plus an IReadOnlyList<IMessageComponentProperties> that mixes
  ActionRow buttons with StringMenu select menus. Also exposes
  BuildModal for the 8 modal-collecting steps.
- DiscordWizardMessenger: IWizardMessenger impl backed by NetCord's
  RestClient + NpgsqlDataSource. Edit falls back to re-send on
  401/403/404. Toast replies are stashed in the existing
  DiscordInteractionReplyCache.
- DiscordWizardSubmitter: 3-retry finalize loop. Builds the shared
  CreateSessionCommand and calls CreateSessionHandler; on success
  edits the message to "ok Created: N sessions", on failure shows
  retry/cancel buttons.
- DiscordWizardCommand: /newsession-wizard slash command with an
  optional mode param (single|pool). Owner/co-GM check via the
  shared group_managers table.
- DiscordPermissionLookup: small helper that loads DB manager ids
  for a guild.

Program.cs gets 5 new singleton registrations (IWizardDraftRepository,
IWizardContextStore, IWizardMessenger, GameCreationWizard,
DiscordWizardSubmitter). The slash command is auto-discovered by
AddApplicationCommands<SlashCommandInteraction, SlashCommandContext>()
+ AddModules(typeof(Program).Assembly).

Build green. All 85 wizard tests + 95 Discord tests pass.
dotnet format clean.

Open: DiscordWizardInteractionModule (button/modal handlers) is not
yet implemented; the bot starts and /newsession-wizard works to the
point of posting the first embed, but subsequent button clicks won't
be handled. A follow-up commit will add the component-interaction
module.

src/GmRelay.DiscordBot/Program.cs

@@ -1,5 +1,6 @@
 using GmRelay.DiscordBot;
 using GmRelay.DiscordBot.Features.Sessions;
+using GmRelay.DiscordBot.Features.Sessions.Wizard;
 using GmRelay.DiscordBot.Infrastructure;
 using GmRelay.DiscordBot.Infrastructure.Discord;
 using GmRelay.DiscordBot.Infrastructure.Health;
@@ -10,6 +11,7 @@ using GmRelay.Shared.Features.Notifications;
 using GmRelay.Shared.Features.Reminders.SendJoinLink;
 using GmRelay.Shared.Features.Reminders.SendOneHourReminder;
 using GmRelay.Shared.Features.Sessions.CreateSession;
+using GmRelay.Shared.Features.Sessions.CreateSession.Wizard;
 using GmRelay.Shared.Features.Sessions.RescheduleSession;
 using GmRelay.Shared.Infrastructure.Scheduling;
 using GmRelay.Shared.Platform;
@@ -82,6 +84,19 @@ builder.Services.AddHostedService<SessionSchedulerService>();
 builder.Services.AddHostedService<DiscordRescheduleVotingDeadlineService>();
 builder.Services.AddHostedService<DiscordHealthCheckHostedService>();
 
+// ── Wizard services (issue #112) ──────────────────────────────────────
+// The Discord wizard reuses the platform-neutral state machine in
+// GmRelay.Shared (GameCreationWizard, IWizardMessenger,
+// IWizardDraftRepository) and only adds a Discord-specific messenger,
+// step renderer, slash command, and submitter on top. The wizard's
+// cleanup service is shared with the Telegram bot and is not
+// registered here — it would compete on the same drafts table.
+builder.Services.AddSingleton<IWizardDraftRepository, GmRelay.Shared.Features.Sessions.CreateSession.Wizard.WizardDraftRepository>();
+builder.Services.AddSingleton<IWizardContextStore, DiscordWizardContextStore>();
+builder.Services.AddSingleton<IWizardMessenger, DiscordWizardMessenger>();
+builder.Services.AddSingleton<GmRelay.Shared.Features.Sessions.CreateSession.Wizard.GameCreationWizard>();
+builder.Services.AddSingleton<DiscordWizardSubmitter>();
+
 builder.Services
     .AddDiscordGateway(options =>
     {