feat: seed Homebrew formula + auto-update workflow

Mirror of installer/homebrew/fai.rb in fai/platform. Source
of truth is the platform repo; this is a passive distribution
mirror.

Auto-update workflow at .forgejo/workflows/update.yml runs
every 4 hours, diffs the latest platform release against
Formula/fai.rb, and rewrites version + sha256 when a new
version ships. Manual trigger via workflow_dispatch.

Users install with:
  brew tap fai/tap https://git.flemming.ai/fai/homebrew-tap
  brew install fai

End-state matches Phase 1 of
docs/releases/go-live-checklist.md.

README.md

@@ -0,0 +1,25 @@
+# homebrew-tap
+
+Distribution mirror for the [F∆I Platform](https://git.flemming.ai/fai/platform).
+
+This repo holds one file — `Formula/fai.rb` — which is a copy of
+[`installer/homebrew/fai.rb`](https://git.flemming.ai/fai/platform/src/branch/main/installer/homebrew/fai.rb)
+in the platform repo. The platform repo is the source of
+truth; this repo exists as a separate target so end users can
+add it as a tap / bucket / channel without subscribing to
+the whole platform tree.
+
+Updates land here automatically via
+`.forgejo/workflows/update.yml` — a 4-hourly cron that
+diffs the latest platform release against this repo's
+manifest and rewrites it when a new version ships. To force
+an immediate update, trigger the **Workflow_dispatch** action
+in the Forgejo UI.
+
+When the workflow itself or the underlying manifest format
+changes, re-seed from the platform repo by running
+`scripts/bootstrap-distribution-repos.sh`.
+
+Issues, PRs, and ad-hoc questions belong at
+`https://git.flemming.ai/fai/platform` — this repo is intentionally
+quiet.