Create GitHub Issue Templates

To help contributors provide all the necessary information when opening issues, we should create standard templates for bug reports and feature requests.

**Acceptance Criteria**
A new .github/ISSUE_TEMPLATE directory is created. Inside, create two markdown files:
-
- **bug_report.md**: This template should have sections for "Describe the bug," "Steps to Reproduce," "Expected Behaviour," "Screenshots," and "Environment (OS, Python version, etc.)."
- **feature_request.md**: This template should have sections for "Is your feature request related to a problem?" "Describe the solution you'd like," and "Describe alternatives you've considered."

Create a .github/ISSUE_TEMPLATE directory in the repository root. Inside, add bug_report.md and feature_request.md files with the sections described in the acceptance criteria. Check existing GitHub issue templates in other projects for formatting examples. Verify the templates appear when opening a new issue on GitHub.

索引モデルが issue の本文から書いたものです。