Pin the third-party Homebrew installer in the bootstrap script

[d3mlabs-ai-flow]

Part of d3mlabs/plans#5.

[JPDuchesne]

Rescoped 2026-08-05 from "pin dev bootstrap fetches to a release SHA instead of live main". The bootstrap script has three fetches, and they don't deserve the same treatment (own-code vs third-party — same model as ai-flow#60):

1. Consumers curling the script from dev/main — stays on main, deliberately. This is our own code, and main is now genuinely review-gated: org ruleset 19118088 requires PR + 1 approving review with no force-push/deletion, and the d3mlabs-ai-flow Integration bypass was removed (verified via API 2026-08-05 — only OrganizationAdmin remains as a bypass actor). A SHA pin here would only defend against an attacker who can already land a reviewed PR on the default branch — at which point the pin gets bumped to the poisoned version anyway — and it would cost auto-deploy: every dev change would need coordinated Dockerfile bumps in every consumer.
2. The Homebrew/install HEAD fetch — the actual gap, pinned in Pin the Homebrew installer to a commit SHA (dev#99) #100. Third-party code piped to bash on a mutable ref. Homebrew/install publishes no tags, so the pin is a commit SHA, bumped deliberately; a stale pin fails benign (it still installs current Homebrew).
3. brew install d3mlabs/d3mlabs/dev — already pinned. The tap formula carries a per-release url + sha256, and releases auto-flow via the formula bump.

Follow-up for consistency (not this repo): cellbound-3d's ci.yml / build-check.yml "Install Homebrew + dev" steps curl the same Homebrew installer unpinned.