[FEATURE] Display SessionStart hook systemMessage in VS Code extension

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When using Claude Code in VS Code, systemMessage output from SessionStart hooks is not visible to users. In the CLI, these messages appear correctly:

SessionStart:startup says: Session started: December 25, 2025 at 01:25:49 PST

However, the VS Code extension doesn't render them in the chat panel. This makes it impossible to verify hooks are working or see useful context they provide.

Proposed Solution

Display systemMessage from SessionStart hooks in the VS Code extension UI, similar to how they appear in the CLI. This could be:

• A dismissible banner at session start
• A line in the chat panel before the first message
• An entry in a "Session Info" section

Alternative Solutions

• Currently using verbose mode (Ctrl+O) but SessionStart messages don't appear there either
• The additionalContext is injected for Claude, but users have no visibility

Priority Level

Low - Nice to have

Feature Category

IDE integrations (VS Code)

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

I have a hook that injects the current datetime at session start. I want to visually confirm the hook is running and see the session start time, especially useful when returning to VS Code after being away.

Additional Context

The hook outputs valid JSON with both additionalContext (for Claude) and systemMessage (for user visibility):

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "<DateTime>...</DateTime>"
  },
  "systemMessage": "Session started: December 25, 2025 at 01:25:49 PST"
}

This works correctly in the CLI but the VS Code extension appears to ignore the systemMessage field for SessionStart hooks.