Make /init customizable

What feature would you like to see?

Allow users to control what /init generates and prevent accidental inclusion of local, potentially sensitive content. In particular:

1. Support a user-defined template for /init (which is currently based on codex-rs/tui/prompt_for_init_command.md
2. Use safer defaults that do not pull from local files (e.g., ~/.codex/AGENTS.md) unless explicitly opted in.
3. Show a clear warning and review step if local content is detected.

This is related with #2529 , which can quickly fix bullets 2 and 3.

Are you interested in implementing this feature?

Possibly — time permitting. Earliest availability is next week.

Additional information

This reduces the risk of leaking personal or org data and aligns with the principle of least surprise. It also complements a related bug report about /init encouraging inclusion of content that may expose sensitive information.