Add .github/copilot-instructions.md for Copilot coding agent

Co-authored-by: MarkusNeusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

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+# Copilot Instructions
+
+This file provides guidance to GitHub Copilot when working with code in this repository.
+
+## Project Overview
+
+**pyplots** is an AI-powered platform for Python data visualization that automatically discovers, generates, tests, and maintains plotting examples. The platform is specification-driven: every plot starts as a library-agnostic Markdown spec, then AI generates implementations for all supported libraries.
+
+**Supported Libraries** (8 total):
+- matplotlib, seaborn, plotly, bokeh, altair, plotnine, pygal, highcharts
+
+**Core Principle**: Community proposes plot ideas via GitHub Issues → AI generates code → Multi-LLM quality checks → Deployed.
+
+## Development Setup
+
+```bash
+# Install dependencies (uses uv - fast Python package manager)
+uv sync --all-extras
+
+# Run all tests
+uv run pytest
+
+# Check code formatting and linting
+uv run ruff check .
+
+# Auto-fix issues
+uv run ruff check . --fix
+
+# Format code
+uv run ruff format .
+```
+
+### Frontend Development
+
+```bash
+cd app
+yarn install
+yarn dev          # Development server
+yarn build        # Production build
+```
+
+## Architecture
+
+### Specification-First Design
+
+Every plot follows this flow:
+```
+specs/{spec-id}.md → plots/{library}/{plot-type}/{spec-id}/default.py
+```
+
+Example:
+```
+specs/scatter-basic.md  → plots/matplotlib/scatter/scatter-basic/default.py
+                        → plots/seaborn/scatterplot/scatter-basic/default.py
+                        → plots/plotly/scatter/scatter-basic/default.py
+                        → (and 5 more libraries...)
+```
+
+### Spec ID Naming Convention
+
+**Format:** `{plot-type}-{variant}-{modifier}` (all lowercase, hyphens only)
+
+Examples: `scatter-basic`, `scatter-color-mapped`, `bar-grouped-horizontal`, `heatmap-correlation`
+
+### Directory Structure
+
+- **`specs/`**: Library-agnostic plot specifications (Markdown)
+- **`plots/{library}/{plot_type}/{spec_id}/{variant}.py`**: Library-specific implementations
+- **`core/`**: Shared business logic (database, repositories, config)
+- **`api/`**: FastAPI backend (routers, schemas, dependencies)
+- **`app/`**: Next.js frontend (React + TypeScript + Vite + MUI)
+- **`rules/`**: Versioned rules for AI code generation and quality evaluation
+- **`tests/unit/`**: Unit tests mirroring source structure
+- **`docs/`**: Architecture and workflow documentation
+
+## Code Standards
+
+### Python Style
+
+- **Linter/Formatter**: Ruff (enforces PEP 8)
+- **Line Length**: 120 characters
+- **Type Hints**: Required for all functions
+- **Docstrings**: Google style for all public functions
+- **Import Order**: Standard library → Third-party → Local
+
+### Testing Standards
+
+- **Coverage Target**: 90%+
+- **Test Structure**: Mirror source structure in `tests/unit/`
+- **Naming**: `test_{what_it_does}`
+- **Fixtures**: Use pytest fixtures in `tests/conftest.py`
+
+### Plot Implementation Template
+
+Every implementation file should:
+1. Start with docstring describing spec ID, library, variant
+2. Define `create_plot()` function with type hints
+3. Validate inputs first (empty data, missing columns)
+4. Use sensible defaults (`figsize=(10, 6)`, `alpha=0.8`)
+5. Include grid for readability
+6. Return the Figure object
+
+### Anti-Patterns to Avoid
+
+- No `preview.png` files in repository (use GCS)
+- No `quality_report.json` files (use GitHub Issues)
+- No hardcoded API keys (use environment variables)
+
+## Tech Stack
+
+- **Backend**: FastAPI, SQLAlchemy (async), PostgreSQL, Python 3.10+
+- **Frontend**: Next.js 14, TypeScript, Tailwind CSS, MUI 7
+- **Package Manager**: uv (Python), yarn (Node.js)
+- **Linting**: Ruff (Python)
+
+## Acceptance Criteria
+
+Before completing any task:
+1. All tests pass: `uv run pytest`
+2. Code passes linting: `uv run ruff check .`
+3. Code is properly formatted: `uv run ruff format --check .`
+4. Type hints are included for all new functions
+5. Docstrings follow Google style for public functions