feat: add install-sgpt-pkg.sh script - with keybindings, auto-update, uninstaller

[bitranox]

• Introduce a comprehensive shell script to install, update, and uninstall sgpt on Ubuntu/Debian systems.
• Configure key binding (Ctrl+L) for quick access to Shell-GPT functionality.
• Automatically fetch the latest release from GitHub and handle version comparisons.
• Patch system-wide shell configuration files (/etc/profile and /etc/bash.bashrc) to integrate sgpt settings.
• Include command-line options for force installation, uninstallation, and displaying help.

This commit adds robust automation and system integration for Shell-GPT, ensuring a smooth user experience on supported systems.

After installation, sgpt can be used with different presets defined in a user-specific configuration file. Each preset can include:

• Custom API endpoint (e.g., OpenAI, OpenRouter),
• Model name (e.g., gpt-4.1-mini, claude-3.7-sonnet),
• User-specific API key,
• Additional flags (e.g., for streaming, long context).

This allows every user on the system to define and use their own AI setup — with complete flexibility over providers, models, context lengths, and access keys.

▶️ Installation

# install or update to the latest sgpt
sudo python3 install-sgpt-pkg.py [OPTION]

Options:

Option	Description
--help	Show usage instructions
--force-install	Force installation of the latest version
--uninstall	Uninstall sgpt and remove all configuration and integration files

🛡️ Security Notes

• File permissions are set strictly (chmod 0600 for config, 0755 for scripts)
• No default API keys included
• User is prompted to edit the config manually
• config survives updates and new installations
• environment is not bloated with keys
• every user on the system can have a different configuration

After install You might login into a new terminal and use :

# Input:
show all files > 8kB <cntrl-L>
# Output :
find . -type f -size +8k
# which You can exec just by pressing enter

# using a preset configuration (otherwise preset "default" is used.
sgpt -p mini "what is the mass of the sun"
sgpt -p claude "what is the mass of the sun"