test(cli): cover rm purge help output

[overtrue]

Summary

The recent rm --purge addition introduced a new user-facing flag, but the CLI help contract did not assert that the flag remained discoverable. That left a small regression window where the option could disappear from rc rm --help without a focused test catching it.

This change adds a single help-contract assertion in crates/cli/tests/help_contract.rs so rc rm --help must continue to include --purge. The scope stays limited to the recently changed rm surface and does not alter production behavior.

Root Cause

PR #82 added the --purge flag to the command implementation, but the help contract for rm still only checked the older options. The command behavior was tested, while the public discoverability contract for the new flag was not.

Validation

This repository does not currently provide a make pre-commit target, so I ran the equivalent required checks directly:

• cargo test --test help_contract top_level_command_help_contract -- --exact
• cargo fmt --all --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace