Decide on and stand up a Homebrew tap for macOS installs

Context

The macOS support work in #3174 (and the follow-up PRs #3942–#3951) makes INSTALL.sh and the GitHub Action installer (actions/instrument/shared/install.sh) try Homebrew first on Darwin, then fall back to a tarball install if brew isn't present or the tap install fails:

brew tap plengauer/opentelemetry-shell https://github.com/plengauer/Thoth
brew install opentelemetry-shell

That tap — plengauer/homebrew-opentelemetry-shell — doesn't exist yet, so today this always falls through to the tarball path. That's fine functionally (the tarball path is the one actually exercised by CI's macos-latest smoke test), but Homebrew is the more idiomatic macOS install experience and is worth having as the primary path eventually.

What's already in place

• meta/homebrew/opentelemetry-shell.rb — a working formula. build-brew (in build.yml) already fills in its __VERSION__/__SHA256__ placeholders on every build and uploads the result as a homebrew-formula.<ref> artifact — it's just not published anywhere yet.
• INSTALL.sh and actions/instrument/shared/install.sh both already degrade gracefully without the tap (brew attempt → tarball fallback), so nothing is currently blocked on this.

What's still needed (needs your call)

1. Create the tap repo. Homebrew taps are separate repos (homebrew-<name>) containing just the formula file(s) — this would be a new plengauer/homebrew-opentelemetry-shell repo. Not something to do unilaterally since it's new infra under the plengauer org.
2. Decide on a publish step. Once the repo exists, a release-time CI step would need to push the templated formula (already built as the homebrew-formula.<ref> artifact) into it — not automated yet.
3. Apple Silicon prefix mismatch. The formula's caveats block already documents this: the shell scripts look for their own files under the fixed path /usr/local/share/opentelemetry_shell, but Homebrew's prefix on Apple Silicon is /opt/homebrew, not /usr/local. Right now that means a real brew install on Apple Silicon would need a manual symlink to work. Worth deciding whether to fix this properly (e.g. via the OTEL_SHELL_HOME override added in meta/debian/postinst, or by having the formula itself create the symlink in post_install) before actually standing up the tap, so the first real users don't hit it.

Opening this as an issue rather than folding it into one of the open PRs since none of them actually touch this — it's new external infrastructure and a product decision, not a code change to this repo.

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