feat: add terminal completion support

[fredvisser]

Adds comprehensive shell completion functionality to SystemLink CLI with support for bash, zsh, fish, and PowerShell.

Changes:

• New module: completion_click.py - Complete shell completion system (352 lines)
• Multi-shell support: Native completion for bash, zsh, fish, and PowerShell with auto-detection
• Installation script: install_completion.py - Standalone completion installer (191 lines)
• PowerShell support: Custom completion script with intelligent command/option completion
• Comprehensive docs: shell-completion.md - Installation guide and troubleshooting (187 lines)
• CLI integration: New slcli completion command with --install and --shell options
• Documentation: Updated README with completion examples and quick setup

Key Features:

• Auto-detects current shell (SHELL env var, Windows detection)
• Completes commands, subcommands, options, and option values
• Cross-platform installation (Windows, macOS, Linux)
• Handles permission errors gracefully with fallback directories
• Consistent with existing CLI patterns and coding standards

[Copilot]

Pull Request Overview

This PR adds comprehensive shell completion functionality to SystemLink CLI, supporting bash, zsh, fish, and PowerShell shells with auto-detection capabilities.

Key changes:

• Implements a complete shell completion system with multi-shell support and automatic installation
• Adds comprehensive documentation and installation scripts for easy setup
• Integrates completion commands into the main CLI interface

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
slcli/main.py	Registers the new completion command with the main CLI
slcli/completion_click.py	Core completion module with shell detection, script generation, and installation logic
scripts/install_completion.py	Standalone installation script for shell completion setup
docs/shell-completion.md	Comprehensive documentation covering installation, usage, and troubleshooting
README.md	Updates main documentation with completion setup instructions