style(lint): enable clippy::single_match_else

[tupe12334]

What

Promotes clippy::single_match_else to deny (the repo's incremental lint ratchet) and fixes the single existing violation.

Why

In run_clean (src/bin/main.rs), find_repo_root_from was destructured with a match whose Some arm was trivial and whose None arm returned early. That's exactly the shape single_match_else flags. Rewritten as an idiomatic let ... else, which reads as the guard it is:

let Some(root) = find_repo_root_from(dir) else {
    println!("steplock: no .steplock/ directory found — nothing to clean");
    return Ok(());
};
let steplock_dir = root.join(".steplock");

Checks

• cargo clippy --all-targets — clean
• cargo test — 15 passed
• cargo fmt --check — clean

Closes #158

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This pull request was opened by the Open issues → fix PRs (owned repos + my orgs) routine of moadim.