refactor: add platform messenger contracts

Introduce platform-neutral PlatformKind, PlatformUser, PlatformGroup, and IPlatformMessenger contracts in GmRelay.Shared.

Route Telegram session schedule updates, direct notifications, interaction replies, and calendar export through TelegramPlatformMessenger while preserving existing Telegram behavior.

Bump version -> 2.0.1

src/GmRelay.Bot/Features/Notifications/DirectSessionNotificationSender.cs

@@ -1,12 +1,12 @@
-using Telegram.Bot;
-using Telegram.Bot.Types.Enums;
+using GmRelay.Bot.Infrastructure.Telegram;
+using GmRelay.Shared.Platform;
 
 namespace GmRelay.Bot.Features.Notifications;
 
 public sealed record DirectNotificationRecipient(long TelegramId, string DisplayName);
 
 public sealed class DirectSessionNotificationSender(
-    ITelegramBotClient bot,
+    IPlatformMessenger messenger,
     ILogger<DirectSessionNotificationSender> logger)
 {
     public async Task SendAsync(
@@ -20,11 +20,11 @@ public sealed class DirectSessionNotificationSender(
         {
             try
             {
-                await bot.SendMessage(
-                    chatId: recipient.TelegramId,
-                    text: htmlText,
-                    parseMode: ParseMode.Html,
-                    cancellationToken: ct);
+                await messenger.SendPrivateMessageAsync(
+                    new PlatformPrivateMessage(
+                        TelegramPlatformIds.User(recipient.TelegramId, recipient.DisplayName),
+                        htmlText),
+                    ct);
             }
             catch (Exception ex)
             {