feat: add checkout lifecycle commands

[laynepenney]

Summary

• add gr checkout list for cache-backed child checkouts
• add gr checkout remove <name> for child checkout cleanup
• extend checkout help text and CLI tests for the new lifecycle commands

Testing

• cargo fmt --all
• cargo test --test cli_tests test_checkout_help_mentions_add_mode -- --nocapture
• cargo test --test cli_tests test_checkout_list_shows_materialized_checkouts -- --nocapture
• cargo test --test cli_tests test_checkout_remove_deletes_materialized_checkout -- --nocapture
• cargo test --test cli_tests test_checkout_remove_errors_for_missing_checkout -- --nocapture

Closes #488.