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/GmRelay.DiscordBot.csproj

@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk.Worker">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>preview</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <UserSecretsId>dotnet-GmRelay.DiscordBot-issue-26</UserSecretsId>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Aspire.Npgsql" Version="13.2.2" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.5" />
+    <PackageReference Include="NetCord.Hosting" Version="1.0.0-alpha.489" />
+    <PackageReference Include="NetCord.Hosting.Services" Version="1.0.0-alpha.489" />
+    <PackageReference Include="Npgsql" Version="10.0.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\GmRelay.ServiceDefaults\GmRelay.ServiceDefaults.csproj" />
+    <ProjectReference Include="..\GmRelay.Shared\GmRelay.Shared.csproj" />
+  </ItemGroup>
+</Project>