[P5] Write NUKE → Fallout migration guide

[ChrisonSimtian]

Scope

User-facing documentation describing how to migrate an existing NUKE consumer project to Fallout.

Outline

1. Why the rename — short, dignified, links to hard-fork announcement
2. What changes — table of `Nuke.* → Fallout.*` package names + namespaces
3. Step-by-step (assumes codefix shipped):
   • Update PackageReference: `Nuke.Common` → `Fallout.Common`
   • dotnet restore
   • Open in IDE, accept the codefix on every `using` directive
   • Update bootstrap scripts if they invoke `dotnet nuke` → `dotnet fallout`
   • Run `./build.ps1` to verify
4. Common gotchas:
   • `.nuke/` directory (depending on [P2] decision)
   • CI YAMLs may be auto-regenerated — review the diff
   • Custom `[Nuke...]` attributes (e.g. `[GitHubActions]`) become `[Fallout...]`?
5. If something breaks — file an issue, what to include in the report
6. Optional — staying on NUKE: link to upstream nuke-build/nuke, note our maintenance scope

Live location

`docs/migration/from-nuke.md` (new), linked from README's "Migrating from NUKE" section.

Done when

• Doc written and reviewed
• At least one external-test reader tries it on a real project
• Linked from README