fix(cli/postinstall): rerun bootstrap when scripts skipped (#369)

[zemaj]

Summary

• export the postinstall bootstrap as runPostinstall and reuse it when the CLI notices missing binaries
• retry runtime bootstrap from coder.js when lifecycle scripts are skipped, while suppressing global alias churn
• add a dry-run unit test so we can exercise the new entry point without fetching real artifacts

Testing

• node --test codex-cli/test/postinstall.test.js
• ./build-fast.sh

Closes #369.