Stand up homebrew-fallout tap with initial formula

Parent: #122

Stand up `Fallout-build/homebrew-fallout` as a Homebrew tap repo and author the initial `Formula/fallout-agent.rb`.

Blocked on RFC #113 — the `fallout-agent` binary/release artifact must exist before the formula can reference a real `url` + `sha256`. Placeholder values are fine until then.

## Definition of done

- Public repo `Fallout-build/homebrew-fallout` exists; README states purpose + install command (`brew tap Fallout-build/fallout && brew install fallout-agent`).
- `Formula/fallout-agent.rb` is valid Ruby; `brew audit --strict --new fallout-agent` passes (or remaining warnings documented).
- Formula references a `url` + `sha256` for the source/bottle (placeholder OK initially; real bottles slotted in by #130).
- Formula declares the `service` block for #132.
- License + maintainer email accurate.