Write the apps snapshot in config init only on macOS (closes #408)

The default config backed up /Applications on every system. That directory exists only on macOS, so on Linux the first snapshot create stopped on the missing path. config init now writes the apps snapshot only when runtime.GOOS is darwin; the macOS file is unchanged.

Model: opus-5-5 (implementation, review); opus-5-5 (merge)

README.md

@@ -32,8 +32,8 @@ mkdir /Volumes/usbstick/mybackup
 # (System Settings → Privacy & Security → Full Disk Access), otherwise
 # the backup will abort with a permission error on protected directories
 
-# run your first backup (the default config backs up ~ and /Applications
-# with sensible excludes)
+# run your first backup (the default config backs up ~, and /Applications
+# on macOS only, with sensible excludes)
 vaultik snapshot create
 
 # see what you have