feat(discord): enable session join leave buttons

Move neutral join/leave handlers into GmRelay.Shared so Telegram and Discord share capacity, waitlist, duplicate-click, and schedule-update behavior.

Add Discord component routing for join_session and leave_session buttons with deferred ephemeral replies and serialized schedule message updates.

Bump version to 2.5.0 and update Discord docs.

Refs #29

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

@@ -77,6 +77,8 @@ public sealed class DiscordStartupTests
         var program = ReadProgram();
         Assert.Contains("DiscordListSessionsHandler", program);
         Assert.Contains("DiscordNewSessionHandler", program);
+        Assert.Contains("JoinSessionHandler", program);
+        Assert.Contains("LeaveSessionHandler", program);
         Assert.Contains("DiscordPermissionChecker", program);
         Assert.Contains("DiscordPlatformMessenger", program);
         Assert.Contains("IPlatformMessenger", program);