fix(cli): set codex process title in launcher

[middleDuckAi]

Summary

• set the Node launcher process title to codex
• keep the launcher surface small by moving the assignment into a dedicated helper
• add an automated Node builtin regression test for the launcher process-title helper

Why

Issue openai#15238 reports that the launcher shows up as node in process listings even though users invoke codex. Setting the launcher process title to codex makes process listings and terminal tooling reflect the actual command being run.

Testing

• node --check codex-cli/bin/codex.js
• node --check codex-cli/bin/process_title.js
• node --check codex-cli/tests/process_title.test.js
• node --test codex-cli/tests/process_title.test.js
• manual process-title repro with a temporary local vendor binary and ps -o pid=,comm=,command=

Notes

• This is intentionally a launcher-only change in codex-cli/bin/codex.js.
• The new test keeps coverage lightweight by using Node's builtin test runner instead of adding extra test infrastructure.