feat: add command for backup/recover brewfile

Signed-off-by: Li Tang <iamtangli42@gmail.com>

brewfiles/README.md

@@ -0,0 +1,28 @@
+# Per-Machine Brewfiles
+
+Each Mac keeps its own Homebrew bundle under `brewfiles/<hostname>/Brewfile`, where `<hostname>` is the short hostname (`hostname -s`, e.g. `stardewvalley`).
+
+## Commands
+
+```bash
+# Dump only (frequent updates)
+./setup.sh brew-backup
+
+# Install packages for this machine
+./setup.sh brew-install
+
+# Full backup still includes brew-backup
+./setup.sh backup
+```
+
+## New machine / full recover
+
+`./setup.sh full-recover` runs `brew-install` automatically when a Brewfile exists for the current hostname.
+
+## Hostname changed?
+
+Point at another machine's Brewfile:
+
+```bash
+BREWFILE_HOST=other-mac ./setup.sh brew-install
+```