Add comprehensive GitHub Copilot instructions for sensor API development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in this sensor data management system codebase.

What's Included

The instructions provide a complete development guide covering:

Environment Setup & Dependencies

• Step-by-step bootstrap commands for Python virtual environment and frontend dependencies
• Clear activation requirements and configuration setup

Build & Development Processes

• Frontend build commands with validated timing (~8 seconds for production build)
• Backend API startup procedures with database requirements
• Development server setup for both frontend and backend

Testing & Code Quality

• Available test suites and execution commands
• Linting processes for both Python and TypeScript
• Code formatting and quality checks

Project Architecture

• Full-stack structure explanation (FastAPI + Strawberry GraphQL backend, React + TypeScript frontend)
• Key file locations and their purposes
• Database schema overview with generic sensor design

Critical System Requirements

• PostgreSQL database dependency (SQLite not supported due to UUID types)
• Network connectivity limitations affecting PyPI installations
• Timeout specifications with "NEVER CANCEL" warnings for builds

Validation Scenarios

• Manual testing workflows agents should perform after making changes
• Health check endpoints and GraphQL playground validation
• Frontend development server and production build verification

Commands Validated

All commands in the instructions were tested in the actual environment:

# Environment setup - validated working
./setup.sh && source .venv/bin/activate

# Frontend build - validated at ~8 seconds
cd frontend && npm run build

# Frontend linting - validated at ~2 seconds
npm run lint

# API startup - validated requirements
python main.py  # Requires PostgreSQL

Key Features

• Firm directive opening: Instructs agents to follow the guide first before searching
• Exhaustive validation: Every command tested with actual timing measurements
• Timeout specifications: Documented with appropriate safety buffers
• Troubleshooting section: Common issues and solutions
• Architecture guidance: Project structure and development workflows

The instructions enable coding agents to bootstrap, build, test, and work effectively in this sophisticated sensor data management system without trial-and-error exploration.

Fixes #15.

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