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

`config init` wrote an `apps` snapshot of `/Applications` on every
system. That directory exists only on macOS, so on Linux the first
`snapshot create` stopped on it unless `--skip-errors` was given.

`defaultConfigText` takes the operating system name and returns the
file with the `apps` snapshot and its comment only for `darwin`;
`config init` passes `runtime.GOOS`. The macOS file is unchanged byte
for byte.

Model: opus-5-5

TODO.md

@@ -24,6 +24,11 @@ owner's; a `v*` tag runs the release build in
 
 # Completed Steps
 
+- 2026-11-10: Made `config init` write the `apps` snapshot, which backs
+  up `/Applications`, only on macOS
+  ([issue #408](https://git.eeqj.de/sneak/vaultik/issues/408)). On
+  Linux the first `snapshot create` stopped on the missing directory.
+
 - 2026-10-15: Stopped `snapshot create --prune` pruning after a backup
   in which a volume mounted below a configured path in an earlier
   snapshot of the same name was not mounted