feat: support project-level .codex config overrides

[tomascupr]

This change adds seamless per-repository overrides of user‐wide Codex settings. Key points:

    * Load sequence:
        1. Global config (`~/.codex/config.{json,yaml,yml}`) as before

        2. Optional project config in `./.codex/config.{json,yaml,yml}`—shallow-merged on top
    * Instructions resolution:
        * If `./.codex/instructions.md` exists, use it; otherwise fall back to `~/.codex/instructions.md`

        * Global instructions file continues to be bootstrapped on first run; we do not auto-create `./.codex`
    * CLI flag `--config` now opens project instructions when present, else global
    * Backward compatibility: existing workflows remain unchanged when no project config is present
    * All tests still pass; formatting and type checks remain green

Please review and let me know if you’d like any tweaks!