feat: support --version flag for @openai/codex-shell-tool-mcp

[bolinfest]

I find it helpful to easily verify which version is running.

Tested:

~/code/codex3/codex-rs/exec-server$ cargo run --bin codex-exec-mcp-server -- --help
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.19s
     Running `/Users/mbolin/code/codex3/codex-rs/target/debug/codex-exec-mcp-server --help`
Usage: codex-exec-mcp-server [OPTIONS]

Options:
      --execve <EXECVE_WRAPPER>  Executable to delegate execve(2) calls to in Bash
      --bash <BASH_PATH>         Path to Bash that has been patched to support execve() wrapping
  -h, --help                     Print help
  -V, --version                  Print version
~/code/codex3/codex-rs/exec-server$ cargo run --bin codex-exec-mcp-server -- --version
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `/Users/mbolin/code/codex3/codex-rs/target/debug/codex-exec-mcp-server --version`
codex-exec-server 0.0.0