fix: the completion subcommand should assume the CLI is named codex, not codex-cli (#1496)

Current 0.4.0 release:

```
~/code/codex2/codex-rs$ codex completion | head
_codex-cli() {
    local i cur prev opts cmd
    COMPREPLY=()
    if [[ "${BASH_VERSINFO[0]}" -ge 4 ]]; then
        cur="$2"
    else
        cur="${COMP_WORDS[COMP_CWORD]}"
    fi
    prev="$3"
    cmd=""
```

with this change:

```
~/code/codex2/codex-rs$ just codex completion | head
cargo run --bin codex -- "$@"
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.82s
     Running `target/debug/codex completion`
_codex() {
    local i cur prev opts cmd
    COMPREPLY=()
    if [[ "${BASH_VERSINFO[0]}" -ge 4 ]]; then
        cur="$2"
    else
        cur="${COMP_WORDS[COMP_CWORD]}"
    fi
    prev="$3"
    cmd=""
```

codex-rs/cli/src/main.rs

@@ -155,6 +155,6 @@ fn prepend_config_flags(
 
 fn print_completion(cmd: CompletionCommand) {
     let mut app = MultitoolCli::command();
-    let name = app.get_name().to_string();
+    let name = "codex";
     generate(cmd.shell, &mut app, name, &mut std::io::stdout());
 }