[GitHubActions] generator: emit CD pipelines (or a thin shell delegating to dotnet fallout Deploy)

Part of #332.

The `[GitHubActions]` generator models "run these targets," not deployment pipelines — so `release.yml`/`experimental.yml`/`preview.yml` are hand-written. Extend it to either:
- emit multi-environment, gated CD pipelines, **or**
- emit a thin generated shell that delegates to `dotnet fallout Deploy` (preferred — keeps the logic in C#, same as validation calls `dotnet fallout Test`).

**Done when** the publish workflows are generated, not hand-authored (modulo the platform approval gate).