fix: guard escapeYamlSingleQuote against undefined input

Default to empty string when value is undefined/null to prevent
replaceAll from throwing on missing CSV fields.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tools/cli/installers/lib/ide/github-copilot.js

@@ -556,7 +556,7 @@ Type \`/bmad-\` in Copilot Chat to see all available BMAD workflows and agent ac
    * @returns {string} Escaped string safe for YAML single-quoted context
    */
   escapeYamlSingleQuote(value) {
-    return value.replaceAll("'", "''");
+    return (value || '').replaceAll("'", "''");
   }
 
   /**