Migrate CLI argument parsing to clap

[hex2dec]

Summary

• migrate leaf command argument parsing in src/cli.rs from the hand-written parser to clap
• preserve the existing command hierarchy, flag names, request construction, and usage/error behavior covered by the current CLI tests
• keep top-level help and subcommand dispatch explicit so the command contract remains stable while the parsing internals change

Validation

• cargo test
• cargo fmt -- --check
• cargo clippy --all-targets --all-features -- -D warnings

Closes #34