Windows Sandbox: Show Everyone-writable directory warning (#6283)

Show a warning when Auto Sandbox mode becomes enabled, if we detect
Everyone-writable directories, since they cannot be protected by the
current implementation of the Sandbox.

This PR also includes changes to how we detect Everyone-writable to be
*much* faster

codex-rs/windows-sandbox-rs/src/logging.rs

@@ -55,3 +55,8 @@ pub fn debug_log(msg: &str, base_dir: Option<&Path>) {
         eprintln!("{msg}");
     }
 }
+
+// Unconditional note logging to sandbox_commands.rust.log
+pub fn log_note(msg: &str, base_dir: Option<&Path>) {
+    append_line(msg, base_dir);
+}