feat: add Homebrew distribution

[edwin-zvs]

Summary

• add a macOS Homebrew formula using the checksummed Apple Silicon and Intel release artifacts
• make the release workflow verify formula version and macOS checksums before publishing
• defer self-updates and automatic update prompts to Homebrew-managed installs
• document tap installation, release maintenance, and the package-manager ownership rule
• record the distribution decision in specs/0168-homebrew-distribution.md

Verification

• cargo test -p construct-cli upgrade
• cargo build --workspace
• rustfmt --edition 2021 --check crates/cli/src/upgrade.rs
• brew style Formula/construct.rb
• ruby -c Formula/construct.rb

The formula is available from this repository as a custom tap immediately after merge. The one-time tap command can be removed if and when the formula is accepted into Homebrew Core.