Add shell completion scripts for bash and zsh

[parfenovvs]

Summary

Generate and install shell completion scripts for bash and zsh to improve command-line user experience.

Features

Completion Support

• Command completion (connect, disconnect, status, etc.)
• Server name completion from cached server list
• Country and city code completion
• Flag and option completion
• Context-aware suggestions

Installation Methods

# Generate completion scripts
mbvpn completion bash > mbvpn-completion.bash
mbvpn completion zsh > mbvpn-completion.zsh

# Install system-wide (requires admin)
mbvpn completion install

# Install for current user
mbvpn completion install --user

Example Completions

$ mbvpn co<TAB>
connect   countries

$ mbvpn connect us-<TAB>
us-east-01    us-east-02    us-west-01    us-west-02

$ mbvpn connect --<TAB>
--auto-reconnect    --kill-switch    --help

Implementation Details

• Use Cobra's built-in completion support
• Dynamic server completion from ~/.config/mbvpn/servers.json
• Completion caching for performance
• Support for both bash and zsh shells

Integration with Package Managers

• Include completion files in deb/rpm packages
• Install to standard locations:
  • Bash: /etc/bash_completion.d/mbvpn
  • Zsh: /usr/share/zsh/site-functions/_mbvpn

Files to Create

• cmd/completion.go - Completion command implementation
• scripts/install-completion.sh - Installation helper
• Update installation documentation

Priority

🎯 Priority 1 (1-2 weeks)

Labels

• enhancement
• priority-1
• user-experience
• developer-experience