[codex] adjust approval and instruction defaults

[viyatb-oai]

Summary:

• Prefer session-scoped default command approval when a prefix suggestion is present.
• Preserve project instruction wrappers when project docs contain reserved markers.

Validation:

• just fmt
• No tests run, per request.

[github-actions]

Closing this pull request because it has had no updates for more than 14 days. If you plan to continue working on it, feel free to reopen or open a new PR.