Agent YAML Parsing Fails with Valid Line Breaks in Frontmatter

Bug Description
When using agents, if there are any valid line breaks in the YAML frontmatter of the agent markdown file, the CLI doesn't recognize that file as existing.

For example, if I have a project file ./claude/agents/code-reviewer.md that starts like this...

---
name: code-reviewer
description:
  Expert code review specialist. Proactively reviews code for quality, security,
  and maintainability. Use immediately after writing or modifying code.
tools: Read, Bash, Glob, Grep
---

You are a senior code reviewer ensuring high standards of code quality and
security.

the CLI won't recognize that it exists, even though this is widely treated as valid formatting, such as by tools like Prettier.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 1.0.62
• Feedback ID: 38b80449-08c7-45ff-b446-3b68b0091d58