As a macOS user I can install the fallout CLI via Homebrew

### Problem
No first-class way to install the `fallout` CLI (the .NET global tool `Fallout.GlobalTools`, command `fallout` — `src/Fallout.Cli`) on macOS. The tap ([Fallout-build/homebrew-fallout](https://github.com/Fallout-build/homebrew-fallout)) exists with a scaffolded `fallout` formula, but it's on placeholders with no anonymous source to install from (#482). This is the CLI counterpart to the agent story #122.

### Outcome
`brew tap Fallout-build/fallout && brew install fallout` installs the CLI on arm64 macOS with no auth and brew-managed upgrades. Formula installs via `depends_on "dotnet"` + `dotnet tool install` (not a self-contained binary).

### Acceptance criteria
- [ ] #482 done — a token-free preview artifact exists.
- [ ] `Formula/fallout.rb` uses a real `url` + `sha256`; `brew install fallout` succeeds on arm64 macOS (net10.0 tool on brewed `dotnet`).
- [ ] Refresh/bump story documented (relates to #133).

### Notes
- Preview is enough to close this; GA can follow later.
- arm64-only for now; Intel dropped, revisit on demand.
- Part of #139. Related: #122 (agent sibling), #482, #129, #133.