feat: add Codex CLI memory cookbook example

[benfrank241]

Summary

Adds a cookbook example for the Hindsight Codex CLI plugin — a hook-based integration that gives OpenAI Codex persistent memory across sessions.

What's included

• README.md — setup guide with architecture diagram, prerequisites, quick start, configuration table, and common issues
• seed_memory.py — standalone script to pre-populate a Hindsight bank with developer facts so recall works immediately
• requirements.txt — hindsight-client>=0.4.19 for the seed script
• .gitignore

How it works

Three hook scripts plug into Codex's hooks system:

1. SessionStart — warms up local Hindsight server
2. UserPromptSubmit — queries Hindsight and injects relevant memories as additionalContext
3. Stop — stores the session transcript to Hindsight for future recall

Test plan

• python seed_memory.py seeds facts successfully against a local Hindsight instance
• README installation steps produce a working hooks setup