fix(cli): missing sha2::Digest import broke the CI build (v0.4.2-beta.3)

Sha256::digest() is a trait method (from digest::Digest, re-exported as
sha2::Digest), not an inherent one - fully-qualifying the call
(sha2::Sha256::digest(...)) doesn't exempt it from Rust's requirement that
the trait itself be in scope for method resolution. Whatever made this
resolve locally without the explicit import didn't reproduce on the CI
runner's dependency resolution, breaking `cargo check` and killing the
whole v0.4.2-beta.3 matrix before a single platform job even started.
Added the import; harmless even where it isn't strictly needed.

ostp/src/main.rs

@@ -3,6 +3,7 @@ use clap::Parser;
 use std::fs;
 use std::path::PathBuf;
 use colored::Colorize;
+use sha2::Digest;
 
 #[derive(Parser, Debug)]
 #[command(author, version, about = "OSTP Core - Ospab Stealth Transport Protocol", long_about = None)]