fix(discord): resolve permission checking for /newsession command

- DiscordPermissionChecker: removed dead-code userRoles overload;
  now only uses resolvedPermissions bitflag (Administrator = 0x8).
- DiscordNewSessionCommand: computes resolved permissions from guild
  user roles via Context.Guild.Users[Id].RoleIds + guild.Roles.
- DiscordNewSessionHandler: updated signature to accept ulong
  resolvedPermissions instead of unused userRoles.
- Added ILogger to command for diagnostics on unexpected errors.
- Added test: regular user with ManageServer (but not Admin) is rejected.

Refs issue #28

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

src/GmRelay.DiscordBot/Infrastructure/Discord/DiscordPermissionChecker.cs

@@ -7,19 +7,8 @@ public sealed class DiscordPermissionChecker
     public bool CanManageSchedule(
         ulong guildOwnerId,
         ulong userId,
-        IEnumerable<ulong> userRoles,
-        IEnumerable<ulong> dbManagerUserIds)
-    {
-        if (userId == guildOwnerId)
-            return true;
-
-        if (dbManagerUserIds.Contains(userId))
-            return true;
-
-        return false;
-    }
-
-    public bool CanManageSchedule(ulong guildOwnerId, ulong userId, IEnumerable<ulong> dbManagerUserIds, ulong resolvedPermissions)
+        IEnumerable<ulong> dbManagerUserIds,
+        ulong resolvedPermissions)
     {
         if (userId == guildOwnerId)
             return true;
@@ -30,3 +19,4 @@ public sealed class DiscordPermissionChecker
         return (resolvedPermissions & AdministratorPermission) == AdministratorPermission;
     }
 }
+