chore: add GitHub issue templates for bug reports and feature requests

Adopts issue template pattern from opencode-antigravity-auth:
- Bug report template with [BUG] prefix and 6-item checklist
- Feature request template with [FEATURE] prefix
- Config disables blank issues, adds links to Google Cloud Support, TOS, and Discussions

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

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,11 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Google Cloud Support
+    url: https://cloud.google.com/support
+    about: For Google Cloud account or billing issues, contact Google Cloud directly
+  - name: Google Terms of Service
+    url: https://policies.google.com/terms
+    about: Review Google's Terms of Service for compliance questions
+  - name: Discussions
+    url: https://github.com/badrisnarayanan/antigravity-claude-proxy/discussions
+    about: Ask questions or discuss the proxy with the community