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/Dockerfile

@@ -0,0 +1,20 @@
+# Stage 1: Build
+FROM mcr.microsoft.com/dotnet/sdk:10.0-noble AS build
+WORKDIR /src
+
+COPY ["src/GmRelay.DiscordBot/GmRelay.DiscordBot.csproj", "src/GmRelay.DiscordBot/"]
+COPY ["src/GmRelay.ServiceDefaults/GmRelay.ServiceDefaults.csproj", "src/GmRelay.ServiceDefaults/"]
+COPY ["src/GmRelay.Shared/GmRelay.Shared.csproj", "src/GmRelay.Shared/"]
+
+RUN dotnet restore "src/GmRelay.DiscordBot/GmRelay.DiscordBot.csproj"
+
+COPY src/ src/
+WORKDIR /src/src/GmRelay.DiscordBot
+RUN dotnet publish "GmRelay.DiscordBot.csproj" -c Release -o /app/publish /p:UseAppHost=false
+
+# Stage 2: Runtime
+FROM mcr.microsoft.com/dotnet/runtime:10.0-noble AS final
+WORKDIR /app
+COPY --from=build /app/publish .
+USER $APP_UID
+ENTRYPOINT ["dotnet", "GmRelay.DiscordBot.dll"]