fix: add parentheses to fix operator precedence in co-author check

[FuturizeRush]

Summary

One-character fix: adds parentheses to correct operator precedence in the co-author line construction.

Bug

src/create-prompt/index.ts:398 — ?? has lower precedence than !==:

// Current (buggy): parses as triggerDisplayName ?? (triggerUsername !== "Unknown")
(githubData.triggerDisplayName ?? context.triggerUsername !== "Unknown")

// Fixed: parses as (triggerDisplayName ?? triggerUsername) !== "Unknown"
((githubData.triggerDisplayName ?? context.triggerUsername) !== "Unknown")

When triggerDisplayName is "" (empty string — not null/undefined), ?? returns "" which is falsy, so the co-author line is incorrectly omitted.

Verified with JavaScript

displayName=""  username="someuser"
  buggy: ""                          ← co-author skipped incorrectly
  fixed: "Co-authored-by: someuser"  ← correct

Test plan

• tsc --noEmit passes
• Tested all 6 input combinations in Node.js — only displayName="" diverges
• Fix is backward compatible (all other cases produce identical output)