Publish a token-free CLI preview artifact for the Homebrew tap

### Problem
The Homebrew tap ([Fallout-build/homebrew-fallout](https://github.com/Fallout-build/homebrew-fallout), #129) has a scaffolded `fallout` formula but no installable source. `Fallout.GlobalTools` is public on GitHub Packages, but GitHub's NuGet registry requires auth even for public packages — anonymous download is `401` (authed works; anonymous doesn't). nuget.org has nothing yet (publish waits for the first `release/2026` tag). So a public `brew install fallout` has nowhere token-free to pull from.

### Outcome
`brew tap Fallout-build/fallout && brew install fallout` installs a **preview** of the CLI on arm64 macOS with no auth — the formula pulls a preview artifact from an anonymously-downloadable source.

### Acceptance criteria
- [ ] Repeatable way to publish a preview CLI artifact downloadable without auth (e.g. nupkg on a public GitHub Release, or a public preview feed).
- [ ] `Formula/fallout.rb` points at it with real `url` + `sha256`; `brew install fallout` succeeds on arm64 macOS (net10.0 tool on brewed `dotnet`).
- [ ] Refresh story documented: how a new preview updates the formula (relates to #133).

### Notes
- arm64-only for now; Intel dropped, revisit on demand.
- Related: #129 (stand up tap), #133 (bump on tag), #122 (agent formula).