migrate-turbo-files

Migrate legacy files in .turbo/ to current formats. Covers plans and shells (layout split, frontmatter normalization, prompt-plan index cleanup) and improvements.md (legacy `trivial`/`standard` Type values rewritten to `direct`/`plan`). Use when the user asks to "migrate turbo files", "migrate turbo", "migrate turboplans", "migrate turbo plans", "upgrade plan format", "add frontmatter to plans", "convert old plans", or "migrate improvements".