SessionStart hook systemMessage no longer rendered in terminal

Description

The systemMessage field returned by a SessionStart hook is no longer displayed in the terminal. Previously it rendered as:

⎿  SessionStart:startup says:
     Gobby Session ID: #4796 (4129ef90-...)
     Agent: default
     ├─ Rules: 72
     └─ Skills: 23

Now the hook executes successfully (exit 0, valid JSON with systemMessage field populated), but nothing is displayed. The additionalContext in hookSpecificOutput IS injected into the model context correctly — only the visual rendering of systemMessage is missing.

Steps to Reproduce

1. Configure a SessionStart hook that returns JSON with a systemMessage field:

{
  "continue": true,
  "systemMessage": "Session started successfully",
  "decision": "approve",
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "some context"
  }
}

1. Start a new Claude Code session
2. Observe: no visual output from the hook, despite additionalContext being injected

Expected Behavior

systemMessage should be displayed in the terminal, as documented ("optional warning message shown to the user").

Environment

• Claude Code: 2.1.88
• OS: macOS (Darwin 25.3.0)
• Terminal: tmux

Related

• #9090 (SessionEnd hook systemMessage not displaying)
• #15344 (Display SessionStart hook systemMessage in VS Code extension)