fix(setup): align braintrust skill name with install path

[Abhijeet Prasad (AbhiPrasad)]

Rename the generated skill frontmatter from braintrust-cli to braintrust so installed skills match the parent directory name used by agent integrations.

Add a setup regression test to ensure future skill renders keep the expected frontmatter name and avoid skill conflict warnings.

pi was giving a warning for this:

[Skill conflicts]
   auto (user) ~/.agents/skills/braintrust/SKILL.md
     name "braintrust-cli" does not match parent directory "braintrust"

[ViaDézo1er / cedric (viadezo1er)]

Thanks, I sometimes had issues with this