feat: implement session hooks for Codex parity

[laynepenney]

Summary

Implement session hooks for Codex so recall can auto-surface journal entries, reminders, and context at session start, similar to Claude Code's hook system.

Why

Layne flagged this in #dev: "we need to make sure that we implement the session hooks for codex in an upcoming sprint." Codex sessions currently start cold without recall context.

Related

• recall#633 (Codex startup parity: auto-surface journal/reminders)
• This may be a sub-task of Codex startup parity: auto-surface journal/reminders like Claude SessionStart #633 or a parallel approach

Boundary

OSS (recall agent lifecycle).