feat(issue-templates): add structured bug & feature issue forms (#3359)

Replace the single runtime-information form with:
- config.yml: disable blank issues, route Q&A/ideas to Discussions, link security policy
- bug-report.yml: reproducible bug form (folds in the old runtime/environment fields + affected-area picker)
- feature-request.yml: scoped proposal form

Uses only default labels (bug/enhancement) so it is self-contained.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,11 @@
+blank_issues_enabled: false
+contact_links:
+  - name: 💬 Questions & usage help
+    url: https://github.com/bytedance/deer-flow/discussions/categories/q-a
+    about: "How do I use X? Why does Y behave like that? Ask in Discussions — it gets answered faster and stays searchable."
+  - name: 💡 Ideas & proposals
+    url: https://github.com/bytedance/deer-flow/discussions/categories/ideas
+    about: Have a half-formed idea? Float it in Discussions before opening a formal feature request.
+  - name: 🔒 Report a security vulnerability
+    url: https://github.com/bytedance/deer-flow/security/policy
+    about: Do not open a public issue for security problems. Follow the security policy instead.