Add shell completion support

[samuel-williams-shopify]

Summary

• add Samovar-local completion support with structured suggestions and TSV result printing
• expose completion through a separate Command.complete entry point defined by the completion concern
• simplify completion input to argv truncated to the cursor, with the final item as the current token
• support explicit native path and directory completion requests via completions: :path, :file, and :directory
• keep shell adapter install/generate behavior out of Samovar and in the companion completion gem

Related

• completion: https://github.com/socketry/completion

Testing

• PATH=/Users/samuel/.local/state/tec/profiles/base/current/global/bin:$PATH BUNDLE_GEMFILE=gems.rb bundle exec sus
• PATH=/Users/samuel/.local/state/tec/profiles/base/current/global/bin:$PATH BUNDLE_GEMFILE=gems.rb bundle exec rubocop