cmds.posix.copy: Allow missing mode option

The -m option of posix copy is optional, and its argparse default is None,
but _run() converts it unconditionally and crashes:

    mode = int(args.mode, 0)

so every copy without -m died with

    TypeError: int() can't convert non-string with explicit base

before the copy even started. Convert the mode only when it is given, and
pass None otherwise, which copy() already treats as "no mode change".

Signed-off-by: Jan Lindemann <jan@janware.com>
Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.99.2

src/python/jw/pkg/cmds/posix/CmdCopy.py

@@ -58,6 +58,7 @@ class CmdCopy(Cmd):  # export
             __expand(args.dst),
             owner = args.owner,
             group = args.group,
-            mode = int(args.mode, 0),
+            # -- A mode is given as a string, if at all
+            mode = int(args.mode, 0) if args.mode is not None else None,
             recursive = args.recursive,
         )