codex-rs had no way to consume the CODE_VERSION the release CI injects, so
`dev --version` printed `codex-cli 0.0.0`. Port hanzo-dev's proven code-version
crate (build.rs reads CODE_VERSION env → rustc-env → option_env! const) as
codex-rs/code-version (package codex-version), wire codex-cli to it, and set clap
name/bin_name/usage/help to `dev` + header to `Hanzo Dev`.
Verified: CODE_VERSION=1.2.3 build → `dev --version` = `dev 1.2.3`; default → `dev 0.0.0`.
This is the only thing codex-rs needed from hanzo-dev before hanzo-dev is removed.
