feat: Config, Command behaviour, and binary discovery

[joshrotenberg]

Summary

Foundation layer for codex_wrapper. Mirror the claude_wrapper architecture:

• CodexWrapper.Config struct: binary path, working_dir, env, timeout, verbose
• Binary discovery: explicit path > CODEX_CLI env var > system PATH
• CodexWrapper.Command behaviour with args/0 and parse_output/2 callbacks
• Generic Command.run/3 that handles System.cmd execution, timeout via Task.async/yield

Acceptance criteria

• Config.new/1 accepts keyword opts, finds codex binary
• Config.cmd_opts/1 returns opts for System.cmd (cd, env, stderr_to_stdout)
• Command behaviour defined with run/3 generic executor
• CodexWrapper.raw/2 escape hatch for arbitrary CLI args
• Tests for config creation, binary discovery fallback chain

Reference

• claude_wrapper: lib/claude_wrapper/config.ex, lib/claude_wrapper/command.ex
• codex-wrapper (Rust): src/lib.rs (Codex builder)