Allow commands to use an instance ID

Following #10, all relevant commands can now use an instance _name_ (e.g., `chazz ssh myinst`). These should also let you use an instance ID, in case you haven't set a name.

This could work by just checking if the name starts with i- and treating it as an ID if so.

This would also let us simplify the `chazz stop` command, which currently has one-off logic to let you use either a name or an ID.

Start by locating the command handlers for the commands that already accept an instance name, then inspect the existing one-off name-or-ID logic in `chazz stop`. Confirm which commands need the same instance-ID behavior and verify that each accepts both an instance name and an ID, with `chazz stop` using the shared behavior.

Written by the indexing model from the issue text.