fix: add parentheses to fix operator precedence in co-author check (#1199)

`??` has lower precedence than `!==`, so the expression:
  triggerDisplayName ?? triggerUsername !== "Unknown"
parses as:
  triggerDisplayName ?? (triggerUsername !== "Unknown")

When triggerDisplayName is an empty string "", the condition
evaluates to "" (falsy), incorrectly skipping the co-author line
even though the user is not "Unknown".

Add parentheses to get the intended behavior:
  (triggerDisplayName ?? triggerUsername) !== "Unknown"

Co-authored-by: Rush <rush@RushdeMacBook-Pro.local>

src/create-prompt/index.ts

@@ -395,7 +395,7 @@ function getCommitInstructions(
   useCommitSigning: boolean,
 ): string {
   const coAuthorLine =
-    (githubData.triggerDisplayName ?? context.triggerUsername !== "Unknown")
+    ((githubData.triggerDisplayName ?? context.triggerUsername) !== "Unknown")
       ? `Co-authored-by: ${githubData.triggerDisplayName ?? context.triggerUsername} <${context.triggerUsername}@users.noreply.github.com>`
       : "";