[pviti]

Part of #189 (Homebrew distribution, tracked step by step there). Draft until the tap repository and HOMEBREW_TAP_TOKEN exist.

Brings back the Homebrew channel removed in 0dba1f4 ("drop the Homebrew channel for now", when the repo was private): brew install commercelayer/tap/rail0.

Stacked on #186: the release job now runs at the repo root, which is where the cask is read from. Merge #186 first; this PR then retargets to main.

What changes

• .goreleaser.yaml: homebrew_casks writes dist/homebrew/Casks/rail0.rb: rail0 plus the r0 alias, bash/zsh/fish completions, man rail0, and a macOS post-install step that lifts the quarantine flag (the binary is not notarized). It uses install_steps, the GoReleaser ≥ 2.19 replacement for the deprecated install.
• release.yml: new step Update the Homebrew tap, after Publish the GitHub release. GoReleaser no longer pushes the cask itself (skip_upload: true): it would push while the release is still a draft, whose download URLs answer 404, so a brew upgrade in that window would get a broken cask. Without HOMEBREW_TAP_TOKEN the step warns and the release goes ahead. An unchanged cask commits nothing, so a resumed run is harmless.
• README: Homebrew is the recommended install, with a note that a brew install upgrades through brew upgrade. rail0 update help and docs say the same.
• selfupdate.ManagedBy: the "dormant" comment is gone. The Caskroom guard (rail0 update → "run brew upgrade rail0") was already there and tested.

Verified locally

• goreleaser check validates; a snapshot release writes the cask.
• The cask, pointed at the published v1.7.0 archives and checksums, installs from a local tap: /opt/homebrew/bin/rail0 and r0 both report rail0 1.7.0; completions and the man page are linked; no com.apple.quarantine on the binary. Then uninstalled and untapped.
• brew style flags 9 layout offenses in the generated file. They only matter for homebrew-core, not for a tap.
• go test ./... passes; docs regenerated.

Before the first release that uses it (owner actions, not done here)

1. Create the public repo commercelayer/homebrew-tap, with a Casks/ directory or empty; the step creates it.
2. Add the repo secret HOMEBREW_TAP_TOKEN to rail0-cli: a fine-grained PAT, or a GitHub App token, with contents: read and write on commercelayer/homebrew-tap only.

The next release (1.7.1 or later) then publishes the cask. Until that release, the README's brew install line finds nothing; that is a reason to release soon after merging, with the tap and the secret already in place.

Also in this PR (homebrew-core preparation, stage 2 of #189)

• packaging/homebrew/rail0.rb: the homebrew-core formula. It builds from source and adds the r0 alias, completions and man pages, plus livecheck and a test do block. Its url points at v1.7.1; the sha256 is filled in on submission.
• bin/brew-formula-test: builds the formula from the working tree in a throwaway local tap and runs brew install --build-from-source, brew test, brew audit --strict and brew style. All four pass locally.
• selfupdate.Packager, which the formula stamps at build time. A packaged build is Homebrew-managed wherever it is installed, so rail0 update points to brew upgrade rail0, and the daily update check is off. Tap and cask installs keep the detection by install path.