feat: Session and SessionServer for multi-turn

[joshrotenberg]

Summary

Multi-turn session support, mirroring claude_wrapper's Session/SessionServer.

• CodexWrapper.Session — in-process session state, threads session_id
• CodexWrapper.SessionServer — GenServer wrapper for long-lived sessions
• Uses codex exec resume under the hood for continuations

Session API

session = CodexWrapper.Session.new(config)
{:ok, session, result} = CodexWrapper.Session.send(session, "implement the feature")
{:ok, session, result} = CodexWrapper.Session.send(session, "now add tests")
CodexWrapper.Session.total_cost(session)

SessionServer API

{:ok, pid} = CodexWrapper.SessionServer.start_link(config: config)
{:ok, result} = CodexWrapper.SessionServer.send_message(pid, "hello")
CodexWrapper.SessionServer.total_cost(pid)
CodexWrapper.SessionServer.history(pid)

Acceptance criteria

• Session.new/2, Session.send/3, Session.stream/3
• Automatic session_id threading via exec resume
• Session.total_cost/1, Session.history/1, Session.turn_count/1
• SessionServer GenServer with start_link/1, send_message/3
• SessionServer status functions: total_cost/1, history/1, etc.
• Tests with mock exec

Depends on

• feat: Exec command and Result struct #2 (Exec command + Result)
• feat: JSON streaming and JsonLineEvent #3 (JSON streaming)

Reference

• claude_wrapper: lib/claude_wrapper/session.ex, lib/claude_wrapper/session_server.ex
• codex-wrapper (Rust): ExecResumeCommand in exec.rs