Add self-update command

[butschster]

This PR adds a new self-update command (with update alias) to the Context Generator CLI, allowing users to automatically update to the latest version without manually downloading the PHAR file.

Features

• Fetches latest release information from GitHub API
• Compares current version with latest available version
• Downloads the new PHAR file if an update is available
• Handles HTTP redirects for GitHub release assets
• Performs platform-specific file operations (Windows vs Unix)
• Provides clear user feedback during the update process
• Requires confirmation before updating
• Supports PSR-18 HTTP clients (requires a client to be available)

Usage

# Check for and apply updates
ctx self-update

# Using the alias
ctx  update

Refactor GitHub API Interaction Architecture

This PR introduces a significant architectural improvement to how the context generator interacts with the GitHub API. The changes extract all API interactions into a dedicated client class, resulting in better separation of concerns, improved testability, and more maintainable code.

Key Changes

1. New GithubClientInterface and implementation:
   • Created a dedicated interface for GitHub API interactions
   • Implemented a concrete client that handles authentication, error handling, and API requests

HTTP Client Refactoring

Problem

Currently, several classes in our codebase need to check if HTTP client dependencies (PSR-18) are available before making HTTP requests. This creates unnecessary duplication and leaks implementation details across the system.

I've refactored the HTTP client implementation to follow better dependency injection principles and the Factory pattern:

1. Created an HttpClientFactory to centralize dependency checks and client instantiation
2. Implemented a NullHttpClient (Null Object pattern) that gracefully handles missing dependencies