[Feature] Allow `specify ... -ai <agent>` to JUST add agent commands

[mcampbell]

As it stands, using specify init ... creates a set of commands for ONE agent. There should be some easy way to create commands either for another agent after the fact, or create the commands for several agents in one go.

If you re-run specify init . --ai <new_agent> this DOES create the commands for the new agent alongside the commands for the initial one, but it also wants to overwrite constitution.md. This is fixable if you have SCM'd your constitution.md file and can easily restore it (git restore ..., for example), but if you haven't or can't, making a copy then copying it back is a bit of a nuisance.

There should be a way to JUST add the commands for a new agent. I understand these are all also available in .zip files to put wherever I want, but this would be a nice QOL CLI improvement.