Fix README install steps; add Linux/macOS install smoke test

[Copilot]

The README told users to run brew install openms/openms/openms without tapping the repo first. The reporter also had to trust the tap before installing. Nothing in CI installed the published bottles on Linux to catch this kind of breakage.

• README

  • The install section now starts with brew tap openms/openms, followed by brew trust openms/openms for setups that require trusting third-party taps.
  • Once the tap is added, the commands use the short formula names.

  brew tap openms/openms
  brew trust openms/openms   # if your Homebrew requires trusting third-party taps
  brew install openms

  • I took brew trust from the reporter's experience and haven't confirmed it against current Homebrew docs. Please check the wording.

• .github/workflows/install-test.yml (new)

  • It taps the repo, installs openms from the published bottles, puts the keg-only bin on PATH and runs FileInfo --help.
  • The matrix covers ubuntu-latest and macos-latest.
  • Triggers: manual dispatch, a daily schedule (06:00 UTC), and pull requests that touch the workflow file, so it runs on this PR.
  • setup-homebrew is referenced at @master with a zizmor ignore, unlike the pinned SHA in tests.yml. Say if you'd prefer it pinned.
  • brew trust is run with || true, so the job doesn't fail on Homebrew versions without that command. That also means the step can't catch a missing or broken trust command.

[jpfeuffer]

@copilot CI fails because of two bugs in this PR:

1. .github/workflows/install-test.yml uses homebrew/actions@master, which does not exist. Use Homebrew/actions/setup-homebrew@1e34f2e2acaa766b7efacb8c26352e6abbb023f9 # 2026.09.13.1, the same pin as tests.yml/build.yml.
2. tap-syntax: shellcheck SC2155 at install-test.yml:29 (export X=$(...)). Declare and assign separately.
   Rebase on the latest main. Verify with brew style openms/openms and actionlint.