Add support for Mistral Vibe CLI

[chris-hatton]

Please add support for mistral.ai's new CLI agent:
https://github.com/mistralai/mistral-vibe

[Fango2007]

By adding a few lines into the specify-cli script it is possible to add partially mistral-vibe support. As there are no templates available for this specific CLI agent, the best option is to force the specify-cli to download codex templates. Afterwards, it is required to rename '.codex' to '.vibe'. Although there are no new /prompts:speckit.* commands, an easy workaround is to use a specific prompt that can trigger the speckit prompt (ex: run the speckit.clarify prompt and strictly follow its instructions.). I did some tests and it seems to work.