feat: Add self-update command

[shahariaazam]

Summary

Implements self-update functionality for gscli to allow users to update the CLI to the latest version from GitHub releases.

Changes

• ✅ New gscli update command to update to latest version
• ✅ New gscli update check command to check for available updates
• ✅ Automatic platform detection (Linux, macOS, Windows)
• ✅ Safe binary replacement with backup mechanism
• ✅ Fetches latest release from GitHub API
• ✅ Version comparison logic
• ✅ Updated README with documentation
• ✅ Updated CHANGELOG

How it works

1. Fetches latest release from GitHub API
2. Compares current version with latest version
3. Downloads appropriate binary for the platform
4. Creates backup of current binary
5. Replaces current binary with new one
6. Removes backup on success or restores on failure

Testing

• ✅ TypeScript compilation passes
• ✅ Help commands display correctly
• ✅ Commands are properly integrated

Usage

# Update to latest version
gscli update

# Check for updates
gscli update check