Add a Homebrew install path for ProfitCtl

[hudsonaikins]

Problem

ProfitCtl now has a working public curl installer and GitHub Releases, but we still only offer go install, source build, or the shell installer. A Homebrew path would reduce friction for the macOS-heavy technical audience that is likely to evaluate the tool first.

Scope

• choose the simplest maintainable Homebrew strategy (brew tap or formula publication)
• document the install and upgrade path
• verify installation from a clean machine flow

Acceptance Criteria

• brew install path is documented and works for the current public release
• upgrade instructions are included
• release docs mention how the Homebrew artifact/formula stays current

[hudsonaikins]

Addressed by PR #32 and merged into main. See #32 for the implementation details.