fix: publish Homebrew cask on first release

[hanneshapke]

Problem

brew install --cask dataiku/tap/kiji-privacy-proxy fails — the tap repo dataiku/homebrew-tap contains only its README.md, no Casks/. The cask has never been published, despite every release running the publish-homebrew-tap job to success.

Root cause is in the "Render cask and push" step. It renders the cask to a brand-new file and then gates the commit on:

if git diff --quiet -- Casks/kiji-privacy-proxy.rb; then
  echo "::notice::Cask already at ${VERSION}, nothing to commit."
  exit 0
fi

git diff --quiet -- <path> only inspects tracked files. On the first publish the cask is untracked, so git reports no change, the step takes the early exit 0, and the git add/commit/push never run. It's chicken-and-egg: it will only commit a file that's already committed.

Confirmed in the v1.1.2 run — the cask rendered with version "1.1.2", then:

##[notice]Cask already at 1.1.2, nothing to commit.

Fix

Stage the file first, then check the staged diff against HEAD:

git add Casks/kiji-privacy-proxy.rb
if git diff --cached --quiet; then
  echo "::notice::Cask already at ${VERSION}, nothing to commit."
  exit 0
fi
git commit ...
git push ...

git diff --cached --quiet detects brand-new files as well as modifications, so the first publish commits the cask and subsequent runs stay idempotent.

After this lands

The next release will populate the tap automatically. To unblock the already-released v1.1.2 immediately without waiting, the tap can be seeded once by committing the rendered cask (version 1.1.2, sha256 bd6a9da0b68befaac6332adfe6f8229058cc5ce66b86b353120e374bb5e5efdc) to dataiku/homebrew-tap.

🤖 Generated with Claude Code

---

Docs (added in this PR)

Documented brew install --cask dataiku/tap/kiji-privacy-proxy as the recommended macOS install path in:

• README.md (Quick Start)
• docs/01-getting-started.md (Quick Installation)
• the generated GitHub Release notes (release.yml)

These instructions become functional once the tap is populated (after this fix ships on the next release, or via a one-time manual seed).