Distribution: go install, Homebrew tap, install.sh, container image

[robbiet480]

Installation is "download a binary from Releases" and nothing else.

Wanted

• go install github.com/CampusTech/fleet-plan/cmd/fleet-plan@latest documented (unblocked once the module rename lands)
• Homebrew tap (CampusTech/homebrew-tap) wired into .goreleaser.yml
• install.sh for CI runners that do not have Go or Homebrew
• Container image published to a registry, for pipelines that prefer a pinned image

Priority: P3