Docs: hookSpecificOutput schema incomplete - missing SessionStart event

Summary

The hook JSON output validation error message shows an incomplete schema for hookSpecificOutput. It only lists three event types, but SessionStart also supports additionalContext injection and works correctly.

Current Schema (from error message)

"hookSpecificOutput": {
  "for PreToolUse": { ... },
  "for UserPromptSubmit": { "hookEventName": "\"UserPromptSubmit\"", "additionalContext": "string (required)" },
  "for PostToolUse": { "hookEventName": "\"PostToolUse\"", "additionalContext": "string (optional)" }
}

Missing Event

SessionStart supports hookSpecificOutput with additionalContext injection, but it's not documented in the schema. This hook works correctly:

const output = {
  hookSpecificOutput: {
    hookEventName: 'SessionStart',
    additionalContext: '...'  // This gets injected into context
  }
};
console.log(JSON.stringify(output));

Impact

When developers see the error message schema, they assume only PreToolUse/UserPromptSubmit/PostToolUse support hookSpecificOutput. This caused confusion when implementing hooks for other events.

Suggested Fix

Update the schema documentation (and error messages) to include all events that support hookSpecificOutput:

• PreToolUse
• UserPromptSubmit
• PostToolUse
• SessionStart (missing)

Environment

• Claude Code CLI
• Pop!_OS 24.04 LTS