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 tu
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 t