Create GSA-TTS/homebrew-tap + acq formula; un-stub installer brew branch

[mogul]

Context

Follow-up from #387 (installer + ADR docs/adr/0026-installation-and-distribution.md, accepted). The installer (install.sh) auto-selects brew -> npm -> clone. The brew branch is currently stubbed: it detects Homebrew, explains the tap isn't published yet, and falls back to the git-clone method.

What this issue tracks

1. Create the external GSA-TTS/homebrew-tap repository.
2. Add an acq formula that installs the runtime footprint (acq + acq.backends/) into libexec and symlinks the acq launcher into bin; declare msb (or its tap) as a dependency so brew install collapses acq + sandbox runtime into one command.
3. Un-stub the brew branch in install.sh so install_via_brew runs brew install GSA-TTS/tap/acq instead of falling back to clone.

Blocking dependency / trigger

Blocked on the GSA-TTS/homebrew-tap repo existing (an external repo that cannot be created from this repo). Per ADR sign-off, this is now unblocked to start. Once the tap + formula are published and validated, un-stub the brew branch and update the README brew note (README.md, currently "coming soon (tap not published yet)").

Acceptance

• brew install GSA-TTS/tap/acq installs a working acq on PATH (Apple Silicon + Intel prefixes).
• install.sh auto-select picks brew when present and installs via the tap (no clone fallback).
• brew upgrade / brew uninstall work; README brew section updated.

Notes

• Rationale and the on-disk-layout constraints (acq sources its acq.backends/ tree relative to a symlink-resolved ACQ_SCRIPT_DIR) are documented in docs/adr/0026-installation-and-distribution.md.

AI-assisted (OpenCode). Human owner: @mogul.