refactor(cli): parse arguments with clap

[tupe12334]

Summary

Follow-up to #209 (review request: "use lib for the cli"). Originally stacked on #210; now rebased onto main after #209 and #210 merged.

• Replaces the hand-written match args.as_slice() in core/src/bin/main.rs with a clap derive parser (Cli plus the CliCommand subcommands). The old print_help is gone. Its checklist-files and global-checklists sections now live in after_help.
• Behaviour is kept:
  • With no subcommand, steplock still handles a hook event from stdin.
  • init [--global], validate, clean [--global], and -V/--version (prints steplock X.Y.Z) work as before.
  • Usage errors still exit 1. clap's default is 2, which steplock uses for hook failures, so main maps it with try_parse and use_stderr().
• New for free: steplock help, steplock <command> --help, and usage errors that name the bad argument.
• MSRV: clap 4.6 needs Rust 1.85, so Cargo.lock pins clap 4.5.61. I resolved it with CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS=fallback. Every new transitive dependency needs Rust ≤ 1.74.
• Lint ratchet: clippy::needless_pass_by_value is now deny.

Note on MSRV (pre-existing, not changed here): real Rust 1.75 can't build steplock even on main. The existing toml 1.1 dependency pulls in serde_spanned 1.1.1, whose manifest Cargo 1.75 fails to parse, and Cargo.lock is lockfile v4, which Cargo 1.75 can't read. The MSRV (1.75) CI job probably still passes because rust-toolchain.toml pins stable, which overrides the toolchain the job installs. Pinning clap to 4.5 keeps this PR from making that worse, but a separate fix is needed, either raising rust-version or pinning the older dependencies.

Test plan

• cargo test passes (6 new parser unit tests, including Cli::command().debug_assert(), and 1 new CLI --help test)
• cargo clippy --all-targets -- -D warnings passes
• cargo fmt -- --check passes
• Manual: steplock --help lists commands and the extra sections; steplock --bogus prints a usage error and exits 1; steplock -V prints steplock 0.1.0

🤖 Generated with Claude Code