Function normalize 

pub fn normalize(
    frontmatter: &Value,
    system_prompt_body: &str,
) -> Result<AgentFile>

Normalize a GitHub Copilot .agent.md file into AgentFile.

Format: YAML frontmatter (---) followed by Markdown body. Frontmatter fields:

• name — agent display name (required)
• description — short description (optional)
• model — LLM model string, e.g. “GPT-4.1”, “claude-sonnet-4-5” (optional)
• tools — list of capability strings like “read”, “github/*” (optional)

The Markdown body after the frontmatter becomes system_prompt.