docs(readme): add Homebrew install instructions

Document the new runfusion/homebrew-fusion tap so users can `brew tap
runfusion/fusion` then `brew install fusion` (or the auto-tap one-liner
`brew install runfusion/fusion/fusion`). Adds a Homebrew block to both
the root README and the CLI README (npm landing page). Gitignore the
sibling clone of the tap repo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/cli/README.md

@@ -20,7 +20,16 @@
 
 ## Install
 
-Zero install, straight from npm:
+**Homebrew** (macOS & Linux):
+
+```bash
+brew tap runfusion/fusion
+brew install fusion
+```
+
+Or as a one-liner: `brew install runfusion/fusion/fusion`.
+
+**npm** — zero install:
 
 ```bash
 npx @runfusion/fusion dashboard