Add contributor workflow templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Report a reproducible problem with Checkjebon.nl
+title: ""
+labels: ""
+assignees: ""
+---
+
+## Description
+
+Describe the problem and the behavior you expected.
+
+## Reproduction
+
+List the steps needed to reproduce the problem.
+
+## Environment
+
+- Browser and version:
+- Device or operating system:
+
+## Additional context
+
+Add screenshots or other useful details. Remove personal shopping-list data before posting.

.github/PULL_REQUEST_TEMPLATE.md

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+## Summary
+
+Describe the change and its user-visible impact.
+
+## Verification
+
+List the browser checks or other verification you performed.
+
+## Checklist
+
+- [ ] I kept the change focused.
+- [ ] I verified the affected workflow in a browser.
+- [ ] I checked a mobile-sized viewport for user-interface changes.
+- [ ] I updated documentation when behavior changed.

CONTRIBUTING.md

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+# Contributing to Checkjebon.nl
+
+Thanks for helping improve Checkjebon.nl.
+
+## Before opening a pull request
+
+- Keep changes focused and explain the user-visible impact.
+- Avoid committing secrets, personal shopping lists, or unrelated generated files.
+- Update `README.md` when supported supermarkets or product behavior change.
+- Describe how you verified the change in the pull request.
+
+## Local verification
+
+This repository is a static website. Serve the repository with a local static-file server, open `index.html`, and verify the affected workflow in a browser.
+
+For user-interface changes, check both a desktop-sized viewport and a narrow mobile-sized viewport.
+
+## Reporting bugs
+
+When reporting a bug, include reproduction steps, the browser and device you used, and a screenshot when it helps explain the issue.