Stand up homebrew-fallout tap with initial formula

[ChrisonSimtian]

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 Build and upload Homebrew bottles to GitHub Releases #130).
• Formula declares the service block for Formula installs launchd plist via brew services #132.
• License + maintainer email accurate.