refactor(client): clean up codex module

[github-actions]

Closes #532

Applied changes based on Issue #532.

I’ve refactored src/client/codex.ts to pull out and simplify the core steps into three small helpers—argument‑building, env‑building, and output‑parsing—and cleaned up the main runCodex flow. Here’s what changed:

• Extracted helpers

  • buildCliArgs(prompt, model, images) uses flatMap for image flags and assembles the full argument list.
  • buildEnvVars(config) centralizes all environment‑variable setup via object spreads.
  • extractCodexOutput(stdout) splits on newlines, finds the second‑last/last timestamp markers via a single reduce, and returns the trimmed block.

• Streamlined runCodex

  • Moved arg/env construction out of the try block and removed manual loops/spreads in favor of the new helpers.
  • Simplified the execa invocation and merged stderr/exit‑code error checks into concise conditionals.
  • Replaced the old fencing + split/map/filter logic with extractCodexOutput(result.stdout).

• Maintained identical behavior
  All existing logging, error types (CliError, TimeoutError), and timestamp‑block logic remain functionally unchanged—just reorganized for clarity and performance.

Please let me know if any tweaks are needed!