[P1] Update README, CLAUDE.md, CONTRIBUTING.md, CODE_OF_CONDUCT.md to Fallout brand

[ChrisonSimtian]

Scope

Cosmetic — text and link updates only, no code changes.

Files

• `README.md` — title, tagline, badges, build-status table, references, Twitter/Slack links
• `CLAUDE.md` — "what this project is" header, links
• `CONTRIBUTING.md` — replace "NUKE is a personal project that was made open-source..." with new ownership story
• `CODE_OF_CONDUCT.md` — references to NUKE

Preserve attribution

In README, add a "Based on NUKE" section credit:

Fallout is the successor to NUKE, originally created by Matthias Koch and many contributors. Fallout continues NUKE's mission as a C#-first build automation framework for .NET.

Badges

NuGet badges need to come down until we have published `Fallout.*` packages on nuget.org. Replace with GitHub Packages badges or remove for now.

Done when

• All four files updated
• Original NUKE attribution prominent in README
• No badges link to broken/wrong URLs