[BUG] additionalContext from SessionStart hook shown to user

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

https://docs.claude.com/en/docs/claude-code/hooks#simple%3A-exit-code
mentions

Exit code 0: Success. stdout is shown to the user in transcript mode (CTRL-R), except for UserPromptSubmit and SessionStart, where stdout is added to the context.

However, that's not what I'm seeing, I see the additional context at startup (context is https://github.com/obra/superpowers/issues/10).

Is this a bug or does the documentation need to be updated?

What Should Happen?

Additional context is not shown to user or docs should be updated.

Error Messages/Logs

Steps to Reproduce

Install https://github.com/obra/superpowers plugin and start claude code

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.14

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

_No response_