[Bug] SessionStart hook additionalContext not persisting to transcript file

Bug Description

SessionStart Hook additionalContext Not Persisting to Transcript

Summary

SessionStart hook's hookSpecificOutput.additionalContext reaches the model during session
nitialization but does not persist to the transcript file on disk.

Expected Behavior

According to the Claude Code hooks API documentation (lines 571-583):

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "My additional context here"
  }
}

The additionalContext field should:

1. Be added to the model's context at session start
2. Persist in the transcript file for the session
3. The option should be available so that it can block the model or not: that is, either the model should immediately begin processing and respond in the block case, or in the default case, add to the context as if a user entered text, but do not trigger model execution.

Actual Behavior

The context is:

• ✅ Successfully generated by the hook (verified in logs)
• ✅ Reaches the model (model can see and respond to the content during the session)
• ❌ Does NOT persist to the transcript JSONL file

Reproduction Steps

1. Configure SessionStart hook to return additionalContext:

sub process {
    my ($self) = @_;
    $self->add_context('The purple squirrel danced gracefully under the moonlit willow tree.');
    return $self->to_hash();
}

1. Start a new Claude Code session (triggers SessionStart hook)

1. Hook output (verified in logs):

{
  "hookSpecificOutput": {
    "additionalContext": "The purple squirrel danced gracefully under the moonlit willow tree.",
    "hookEventName": "SessionStart"
  }
}

1. Query the model about the purple squirrel - model can see it ✅

1. Check transcript file - context is missing ❌

Environment

• Claude Code hooks API implementation
• SessionStart hook event
• Hook output structure matches API documentation exactly

Impact

SessionStart context is ephemeral - available only during the initial session but lost if Claude is restarted

Related Documentation

• docs/claude-hook-api.md lines 571-583 (SessionStart Decision Control)
• docs/claude-hook-api.md lines 427-428 (Exit code 0 behavior for SessionStart)

Environment Info

• Platform: linux
• Terminal: vte-based
• Version: 2.0.37
• Feedback ID: ac21ca3a-e938-4c92-93cc-c782158a338d

Errors

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