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>

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

@@ -11,8 +11,8 @@ public sealed class DiscordPermissionCheckerTests
         var result = checker.CanManageSchedule(
             guildOwnerId: 123456789ul,
             userId: 123456789ul,
-            userRoles: Array.Empty<ulong>(),
-            dbManagerUserIds: Array.Empty<ulong>());
+            dbManagerUserIds: Array.Empty<ulong>(),
+            resolvedPermissions: 0);
 
         Assert.True(result);
     }
@@ -38,8 +38,8 @@ public sealed class DiscordPermissionCheckerTests
         var result = checker.CanManageSchedule(
             guildOwnerId: 123456789ul,
             userId: managerId,
-            userRoles: Array.Empty<ulong>(),
-            dbManagerUserIds: new[] { managerId });
+            dbManagerUserIds: new[] { managerId },
+            resolvedPermissions: 0);
 
         Assert.True(result);
     }
@@ -51,8 +51,21 @@ public sealed class DiscordPermissionCheckerTests
         var result = checker.CanManageSchedule(
             guildOwnerId: 123456789ul,
             userId: 111ul,
-            userRoles: Array.Empty<ulong>(),
-            dbManagerUserIds: new[] { 222ul });
+            dbManagerUserIds: new[] { 222ul },
+            resolvedPermissions: 0);
+
+        Assert.False(result);
+    }
+
+    [Fact]
+    public void CanManageSchedule_WhenUserHasOtherPermissionButNotAdmin_ReturnsFalse()
+    {
+        var checker = new DiscordPermissionChecker();
+        var result = checker.CanManageSchedule(
+            guildOwnerId: 123456789ul,
+            userId: 111ul,
+            dbManagerUserIds: Array.Empty<ulong>(),
+            resolvedPermissions: 0x4); // ManageServer, not Administrator
 
         Assert.False(result);
     }