[P6] Rename global tool: dotnet nuke → dotnet fallout

[ChrisonSimtian]

Scope

The `Nuke.GlobalTool` package publishes a `dotnet nuke` CLI command. Rename to `dotnet fallout`.

Files

• `source/Nuke.GlobalTool/Nuke.GlobalTool.csproj` — `nuke` → `fallout`
• After [P4]: project itself renames to `Fallout.GlobalTool`

Migration impact

• Existing scripts invoking `dotnet nuke :setup` / `dotnet nuke :secrets` etc. break.
• Bootstrap scripts (`build.ps1`, `build.sh`, `build.cmd`) reference `dotnet run --project _build.csproj` not `dotnet nuke`, so they're unaffected.
• IDE plugins (ReSharper, Rider, VSCode) reference the command name — coordinate with plugin updates.

Transition strategy

Optionally publish both `Fallout.GlobalTool` (registers `dotnet fallout`) AND a tiny shim `Fallout.Compat.GlobalTool` (registers `dotnet nuke` that invokes `fallout` with a deprecation warning).

Recommend: skip the shim. Document the rename in the migration guide.

Done when

• `dotnet tool install --global Fallout.GlobalTool` registers `fallout` command
• CLI behavior identical to old `nuke` command
• Migration guide section covers the change