feat: Exec command and Result struct

[joshrotenberg]

Summary

The primary interface: CodexWrapper.exec/2 wraps codex exec <prompt>.

• CodexWrapper.Exec struct with builder functions for all exec options
• CodexWrapper.Result struct: stdout, stderr, exit_code, success
• Type definitions for SandboxMode (:read_only, :workspace_write, :danger_full_access)
• Type definitions for ApprovalPolicy (:untrusted, :on_request, :never)

Exec options to support

• prompt (positional, required)
• model, sandbox, approval_policy (ask_for_approval)
• full_auto, dangerously_bypass_approvals_and_sandbox
• cd, skip_git_repo_check, add_dir (list)
• search, ephemeral, output_schema
• json, output_last_message
• image (list), config (list of key=value)
• enable/disable (list)

Convenience API

# Simple
CodexWrapper.exec("implement the feature", working_dir: "/path/to/repo")

# Full control
CodexWrapper.Exec.new("implement the feature")
|> CodexWrapper.Exec.model("o3")
|> CodexWrapper.Exec.sandbox(:danger_full_access)
|> CodexWrapper.Exec.dangerously_bypass_approvals_and_sandbox()
|> CodexWrapper.Exec.execute(config)

Acceptance criteria

• Exec.new/1 and builder functions for all options
• Exec.execute/2 runs codex CLI, returns {:ok, Result.t()} | {:error, term()}
• CodexWrapper.exec/2 convenience function
• SandboxMode and ApprovalPolicy types with string conversion
• Result.from_cmd/2 parses System.cmd output
• Tests with mock binary or integration tests

Depends on

• feat: Config, Command behaviour, and binary discovery #1 (Config + Command behaviour)

Reference

• codex-wrapper (Rust): src/command/exec.rs
• claude_wrapper: lib/claude_wrapper/query.ex, lib/claude_wrapper/result.ex