build(release): publish brew cask to remetric-dev/homebrew-tap on each tag

[cicdteam]

Summary

Wires GoReleaser to publish a Homebrew Cask to a new remetric-dev/homebrew-tap repo on every tag. After this PR merges and the manual setup below is done, the next release tag auto-populates Casks/remetric.rb and users can:

brew install remetric-dev/tap/remetric

GoReleaser v2.10 deprecated brews: in favour of homebrew_casks: for prebuilt-binary distribution; this PR uses the new section directly rather than pinning an older GoReleaser version.

Manual prerequisites (one-time, after merge)

These have to be done by a human with org-admin rights; they can't go in this PR.

1. Create empty tap repo:

   gh repo create remetric-dev/homebrew-tap --public \
     --description "Homebrew tap for remetric" --add-readme

2. Create a fine-grained PAT at https://github.com/settings/personal-access-tokens with:

   • Resource owner: remetric-dev
   • Repository access: remetric-dev/homebrew-tap only
   • Permission: Contents: Read and write
   • No expiration (or as long as practical).

3. Add PAT as repo secret in remetric-dev/remetric:

   • Settings -> Secrets and variables -> Actions -> New repository secret
   • Name: HOMEBREW_TAP_TOKEN
   • Value: the PAT from step 2

4. Tag a release (next v0.1.0-rc.N or v0.1.0). The release workflow runs goreleaser which pushes Casks/remetric.rb to the tap repo.

5. Verify with brew install remetric-dev/tap/remetric && remetric --version.

Test plan

• make release-check (goreleaser v2.10.0) - config validates clean.
• CI green on this PR (only ci workflow runs; release triggers on tag).
• Post-merge + manual setup: next git tag v0.1.0-rc.X && git push origin v0.1.0-rc.X runs release workflow successfully and produces a non-empty Casks/remetric.rb in the tap repo.
• brew install remetric-dev/tap/remetric installs the binary and remetric --version prints the expected version.

Files

• .goreleaser.yml: add homebrew_casks: section pointing at the tap repo.
• .github/workflows/release.yml: pass HOMEBREW_TAP_TOKEN secret into the goreleaser step.
• README.md: add Homebrew install section; drop "No Homebrew tap" from "What's still missing".
• docs/getting-started.md: add Homebrew install line.