#Refactor CLI command dispatch: replace reflection-based partial `Program` god-class with `IFalloutCommand` types

### Problem
`Fallout.Cli` implements every CLI command as a `static int` method on one `partial class Program`, split across ~10 `Program.*.cs` files and dispatched by reflection (`Program.Handle` reflects over its own `int`-returning methods). The partial split looks organized, but the underlying type is a god object — it owns dispatch, secrets, packages, Cake interop, navigation, config, run orchestration, and the Spectre prompts. Reflection dispatch is stringly-typed: a typo'd signature compiles and fails at runtime, and nothing is DI-able or unit-testable in isolation.

### Outcome
One command = one type behind a typed `IFalloutCommand`, discovered by type via DI instead of reflection. The `:command` CLI surface and shell-function names stay exactly as they are (non-breaking).

```csharp
public interface IFalloutCommand
{
string Name { get; } // e.g. "run", "setup"
int Execute(string[] args, AbsolutePath rootDirectory, AbsolutePath buildScript);
}
```

Each `Program.X.cs` partial becomes a standalone `XCommand` implementing `IFalloutCommand`; the dispatcher resolves by `Name`. The `partial` keyword disappears. Shared helpers move into injectable services in the final collapse PR.

### Acceptance criteria
- [ ] `IFalloutCommand` + a type/DI-based dispatcher replace reflection dispatch
- [ ] Every command is its own type; `Program` collapses to a thin entry point (no `partial`)
- [ ] CLI command names + shell-function spellings unchanged (dash/case-insensitive preserved)
- [ ] Dispatch has unit-test coverage
- [ ] Shared helpers extracted into injectable services

### Approach
Incremental — one command per PR behind a transitional `DelegateCommand` adapter so nothing regresses. Tracked by the #394 stack.