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.AppHost/Program.cs

@@ -8,6 +8,10 @@ builder.AddProject<Projects.GmRelay_Bot>("bot")
     .WithReference(postgres)
     .WaitFor(postgres);
 
+builder.AddProject<Projects.GmRelay_DiscordBot>("discord")
+    .WithReference(postgres)
+    .WaitFor(postgres);
+
 builder.AddProject<Projects.GmRelay_Web>("web")
     .WithReference(postgres)
     .WaitFor(postgres);