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

src/GmRelay.DiscordBot/DiscordOptions.cs

@@ -0,0 +1,15 @@
+namespace GmRelay.DiscordBot;
+
+public sealed class DiscordOptions
+{
+    public string? Token { get; init; }
+
+    public void Validate()
+    {
+        if (string.IsNullOrWhiteSpace(Token))
+        {
+            throw new InvalidOperationException(
+                "Discord:Token is required. Set via environment variable Discord__Token or user secrets.");
+        }
+    }
+}