Creates a new draft post with valid frontmatter.

Usage

mix cherry.gen.post "Post title" [--source DIR] [--json]

The file lands in content/posts/ as YYYY-MM-DD-slug.md (today's date, slug derived from the title) with draft: true. Publishing later re-dates it (mix cherry.publish). With --json, the envelope carries the path — agents: write your content into that file, then cherry build --drafts to preview.