- Capture `brew tap` output and print the same diagnostic tail on
failure that `brew install` already prints, so #2867-style "no
signal" reports are gone from both Homebrew failure paths.
- Add a `brew tap` failure regression case to `scripts/install.test.sh`
and refactor the test runner to share sandbox/curl-stub setup; both
cases now also assert the diagnostic tail is emitted.
- Move the shell installer test out of the heavy backend job into a
dedicated `installer` matrix job that runs on `ubuntu-latest` and
`macos-latest`, since the installer targets macOS/Homebrew and BSD vs
GNU `tar` / `sed` / `mktemp` differences are the next likely break.
- Surface `MULTICA_INSTALL_DIR`, `MULTICA_BIN_DIR`, and
`MULTICA_SELFHOST_REF` in `install.sh --help` so `MULTICA_BIN_DIR`
stops looking like a test-only knob.
Co-authored-by: multica-agent <github@multica.ai>
