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Sad to say, Ulysses has no concept of YAML metadata for Markdown files.
If you include it in your sheets, Ulysses will not know what to do with it. For example, the sheet "preview" will not recognize the title — here's a screenshot:
Also, the built-in previewer for HTML, PDF, etc, does not recognize YAML at all.
In fact there's no support at all, anywhere, for the most common kinds of metadata (title, author, date, etc), either per-sheet or per-project. This seems like a glaring omission for an app that purports to fulfill all the needs of novelists and bloggers and journalists.