Remove support for main.agent.md

Now that `debug: true` has been added to agent frontmatter, remove special case for main.agent.md.

Customers can still create an agent named main.agent.md, but it just behaves as any other agent.

Search the repository for the `main.agent.md` special case and read the surrounding agent-loading logic first. Remove the name-specific behavior while preserving ordinary agent handling, then run the existing test suite and verify that an agent named `main.agent.md` behaves like any other agent.

Written by the indexing model from the issue text.