I expect copied files to have the same flags as the original file

I've been copying some executable files and noticed the permission flags are getting dropped on a copy. It would be nice if the library defaulted to maintaining file flags.

My workaround:

```clojure
(defn copy-permissions [from to]
(when (fs/executable? from)
(fs/chmod "+x" to))
(when (fs/writeable? from)
(fs/chmod "+w" to))
(when (fs/readable? from)
(fs/chmod "+r" to)))

(defonce _|HACK_TOWN
(alter-var-root #'fs/copy
(fn [old]
(fn [from to]
(old from to)
(copy-permissions from to)))))
```

Happy to turn it into a PR if others are in agreement about the change in functionality.

Start by reading the fs/copy entry point and trace how copied files are created. Clarify which permission flags should be preserved by default, including the executable, writable, and readable cases shown in the workaround, then verify that copying retains those flags without breaking existing behavior.

Written by the indexing model from the issue text.