Windows: `rtk tree` still corrupts arguments containing a quote — last unconverted sibling after #3728

src/cmds/system/tree.rs:35 forwards rtk's own CLI arguments to a PATH-resolved tree with plain cmd.arg:

for arg in args {
    cmd.arg(arg);
}

#3728 introduced ChildArgExt::child_arg/child_args and converted the rest of this family — ls, wc, find, grep/rg, proxy, run_passthrough and the main.rs unknown-command fallback — but tree was not converted. MSYS2 ships an MSYS-linked tree, so on Windows this reproduces #3727 exactly:

rtk tree -P '*"*'
rtk tree -I 'a"b'

The -I pattern built on line 32 from NOISE_DIRS is rtk-generated and contains no quote, so it is unaffected.

Fix is mechanical — swap the loop for cmd.child_args(args), matching wc_cmd.rs.

I checked the rest of src/cmds/system/ while reviewing #3728: read.rs's Command::new sites are all inside #[cfg(test)], pipe_cmd.rs spawns nothing, and ast_grep/ctest target native binaries. tree.rs is the only remaining gap in this family.

Follow-up from the review of #3728.

Start with src/cmds/system/tree.rs:35 and compare its argument loop with the child_args usage in wc_cmd.rs. Convert tree's forwarded arguments consistently, then verify the quoted -P and -I examples on Windows/MSYS2 no longer corrupt their arguments.