refactor(usage_md): return ExitCode instead of calling process::exit

https://claude.ai/code/session_01TSnvxN1HBwwzhxfFShWAQ5

Author: claude

cli/usage_md.rs

@@ -1,11 +1,15 @@
 use parallel_disk_usage::usage_md::render_usage_md;
+use std::process::ExitCode;
 
-fn main() {
+fn main() -> ExitCode {
     match render_usage_md() {
-        Ok(content) => println!("{}", content.trim_end()),
+        Ok(content) => {
+            println!("{}", content.trim_end());
+            ExitCode::SUCCESS
+        }
         Err(error) => {
             eprintln!("error: {error}");
-            std::process::exit(1);
+            ExitCode::FAILURE
         }
     }
 }