chore(lint): enable clippy::single_match_else

[tupe12334]

Closes #158

Enables clippy::single_match_else (pedantic) as deny in core/Cargo.toml, continuing the incremental Clippy ratchet.

What the lint catches

A two-arm match where one arm is trivial/catch-all + an else block — clearer as if let { } else { } or let … else { }.

Change

• core/Cargo.toml: add single_match_else = "deny".
• core/src/bin/main.rs: rewrite the one violation in run_clean as a let … else, dropping a level of nesting:

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");

Verification (matches CI)

• cargo fmt --manifest-path core/Cargo.toml -- --check ✅
• cargo clippy --manifest-path core/Cargo.toml --all-targets --locked -- -D warnings ✅
• cargo test --manifest-path core/Cargo.toml --locked ✅ (5 passed)
• RUSTDOCFLAGS="-D warnings" cargo doc --manifest-path core/Cargo.toml --no-deps --locked ✅

One lint, one fix, no other lints touched.

🤖 Generated with Claude Code