[SDK-12] Plugin scaffold template — dotnet new fallout-plugin

Ship a `dotnet new` template so plugin authors can scaffold a new plugin project with one command. Standard companion deliverable for any SDK (Roslyn analyzers, MSBuild SDKs, source generators all do this).

## Scope

- New NuGet template package `Fallout.Plugin.Sdk.Templates`.
- Template name: `fallout-plugin`. Invocation: `dotnet new fallout-plugin -n MyCompany.MyPlugin`.
- Generated project includes:
- csproj targeting the same TFMs as `Fallout.Plugin.Sdk`
- `[assembly: FalloutPlugin(typeof(Plugin))]` baseline
- One `IFalloutPlugin` implementation stub with a `Configure(IServiceCollection)` example
- Reference to `Fallout.Plugin.Sdk.Testing` (#159) with one passing smoke test
- README pointing at the plugin-author docs (#160)
- Documented as the recommended starting point on the plugin-author docs page.

## Distinct from #158

#158 is one in-repo reference plugin used for our own smoke-test. This issue is a template authors consume externally to start their own repo. Different deliverable, different lifecycle.

## Depends on

SDK-1 (#146), SDK-2 (#147), SDK-9 testing helpers (#159).

## Done when

- [ ] `dotnet new install Fallout.Plugin.Sdk.Templates` works against nuget.org
- [ ] Scaffolded project builds and its smoke test passes with zero further setup
- [ ] Author docs page (#160) opens with "scaffold via `dotnet new fallout-plugin`"