First-class ReleaseChannel / DeploymentTarget / Environment model

Part of #332.

Introduce code-defined, first-class constructs:
- **ReleaseChannel** — `alpha`/`preview`/`rc`/`stable`: versioning rules + feed routing + maturity policy.
- **DeploymentTarget** — a NuGet feed, GitHub Release, container registry, cloud target (#14–#17), SSH host (#250), etc.
- **Environment** — named, ordered, gated (`dev` → `staging` → `prod`) with promotion + approval semantics.

These become the vocabulary a richer `Publish`/`Deploy` surface (see sibling issue) consumes. RFC the shape (types, how they compose, how channels map to targets per environment).