feat: add issue and PR templates

Add structured issue templates for bug reports and feature requests,
with dropdown menus for channel, LLM provider, Python version, and OS.
Redirect questions to Discussions. Add PR template with checklist.

Ref: https://github.com/HKUDS/nanobot/discussions/3284

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,24 @@
+## Summary
+
+<!-- Brief description of what this PR does -->
+
+## Related Issue
+
+<!-- Link to the issue this PR addresses, e.g., Fixes #123 -->
+
+## Changes
+
+<!-- List the key changes -->
+
+-
+
+## Checklist
+
+- [ ] I have read the [Contributing Guide](../CONTRIBUTING.md)
+- [ ] My changes follow the existing code style
+- [ ] I have tested my changes locally
+- [ ] I have updated documentation if applicable
+
+## Screenshots / Logs (if applicable)
+
+<!-- Add any relevant screenshots or log output -->