feat: Add shell completion support (bash, zsh, fish)

[nurazon59]

Why

gws has a rich set of services, resources, and methods, but there's no shell completion support yet. Tab completion would significantly improve the CLI experience, especially for discovering available services/resources/methods without referring to --help each time.

What

Add shell completion generation for major shells:

• bash
• zsh
• fish

Ideally via a subcommand like gws completion <shell> that outputs the completion script to stdout, following the common CLI pattern (e.g., gh completion, kubectl completion).

How

• Top-level services (drive, sheets, gmail, calendar, etc.)
• Resources per service (files, spreadsheets, messages, etc.)
• Methods per resource (list, get, create, update, delete, etc.)
• Global flags (--params, --json, --format, --page-all, etc.)