feat: add Discord NetCord gateway worker

Add a separate GmRelay.DiscordBot worker using NetCord Gateway with startup token validation, PostgreSQL datasource registration, slash-command setup, component interaction service registration, and lifecycle logging.

Wire the Discord service through Aspire AppHost, Docker Compose, PR checks, deploy image build/push/scan/pull steps, README docs, and synchronized version 2.2.0.

Add TDD coverage for project isolation, token validation, startup wiring, runtime wiring, and version synchronization.

Bump version -> 2.2.0

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

@@ -0,0 +1,28 @@
+using GmRelay.DiscordBot;
+
+namespace GmRelay.Bot.Tests.Discord;
+
+public sealed class DiscordOptionsTests
+{
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void Validate_ShouldRejectMissingToken(string? token)
+    {
+        var options = new DiscordOptions { Token = token };
+
+        var exception = Assert.Throws<InvalidOperationException>(options.Validate);
+
+        Assert.Contains("Discord:Token is required", exception.Message);
+        Assert.Contains("Discord__Token", exception.Message);
+    }
+
+    [Fact]
+    public void Validate_ShouldAcceptConfiguredToken()
+    {
+        var options = new DiscordOptions { Token = "configured-token" };
+
+        options.Validate();
+    }
+}