Transform README from congratulations page to comprehensive project documentation

[Copilot]

The README was a GitHub Skills exercise completion page with no project information. Replaced with proper documentation while preserving the congratulations section at the end.

Changes

• Added project overview: FastAPI application for managing high school extracurricular activities
• Installation & usage: Clone, install, run instructions with API endpoint reference table
• Project structure: Directory tree and file descriptions
• API examples: curl commands for signup/unregister operations
• Testing: pytest invocation and test coverage summary
• Proper hierarchy: H1-H3 headings for GitHub's auto-generated TOC
• Relative links: LICENSE file referenced correctly for offline browsing

Structure

# Title & Description
## Features
## Prerequisites
## Installation
## Usage
  ### Running the Application
  ### API Endpoints
  ### Example API Usage
## Project Structure
## Available Activities
## Testing
## Data Storage
## License
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## 🎉 GitHub Skills Exercise Complete! (preserved)

Information density increased from 37 lines (all marketing) to 171 lines of technical documentation.

Original prompt

Please review and improve our README.md file.

Custom agent used: readme-specialist
Specialized agent for creating and improving README files and project documentation

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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.