[codex] Add memory flag to README config example

[owainlewis]

Summary

• add features.memory: true to the README neo.yaml config example
• use the same project-root memory.md wording as the default config
• keep the README feature flags aligned with the embedded defaults and generated config docs

Verification

• git diff --check
• inspected the README snippet against internal/config/defaults/neo.yaml
• review subagent checked issue scope and acceptance criteria