docs: Add issue template config (disable blank issues, security link)

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

Author: kochj23

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security Vulnerability
+    url: https://github.com/kochj23/RsyncGUI/security/policy
+    about: Report security vulnerabilities privately — do not open a public issue.