Add Copilot instructions for repository

[Copilot]

Configure .github/copilot-instructions.md per GitHub best practices to provide Copilot with project context.

Changes

Created comprehensive Copilot instructions covering:

• Tech stack: Django 5.2.7, DRF, PostgreSQL/SQLite, Redis, Celery, pytest + hypothesis
• Project structure: 11 Django apps (auths, submissions, courses, problems, etc.) and their purposes
• Development environment: 4 concurrent services (Redis, Celery worker, Django server, test terminal)
• Testing conventions: pytest markers, fixtures (api_client), property-based testing profiles
• Git workflow: Branch strategy (main/dev/feature), commit format (type: subject), PR process
• Critical notes: Celery worker restart on code changes, SQLite vs PostgreSQL, JWT auth, sandbox integration

Context

267-line instructions file provides Copilot with:

• Service dependency requirements (Redis for Celery, external sandbox API)
• Key gotchas (Celery auto-reload, environment variables, .gitignore patterns)
• Common commands and troubleshooting
• References to existing docs (docs/developers.MD, app-specific docs)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #167

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.