As of July 1:
The built-in Explore agent now inherits the main session’s model (capped at opus) instead of running on haiku
https://code.claude.com/docs/en/changelog#2-1-198
This is a way to get back the old explore agent. Both files are needed, the skill needs to point to the shell script in the correct directory.
After running the skill you should have ~/.claude/agents/Explore.md. I think the capitalization there is important. You can ask Claude to do smething that uses the explore agent, and have it verify it's using Haiku.