Update check: frequency setting ineffective, notice suggests the wrong command #19

Found during #17 review.

• main._check_for_updates_if_needed (~61-65) records the check timestamp only when an update is found, so update_check_frequency has little effect (the 24-hour PyPI cache from refactor: enforce maximum strictness across the codebase #17 limits the damage).
• The notice suggests kilm update, which only prints help; it should say kilm update perform.
• An unknown latest version is reported as up to date.
• The auto_update setting and the force_check parameter are never used: implement or remove.
• The root --help runs the update check; help output should not depend on the network.