chore(vibe): Add `.vibe` config with custom prompt

[github-actions]

Note

The pull request "chore(vibe): Add .vibe config with custom prompt" was created by @s1gr1d but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Add config and default custom prompt for the repository.

Docs: https://docs.mistral.ai/mistral-vibe/introduction/configuration

[linear]

JS-1631