migrate-turboplans

Migrate legacy plan and shell files in .turbo/ to the current format with YAML frontmatter. Handles three formats: inline prompt plans (.turbo/prompt-plans/ or .turbo/prompts.md with prompts embedded in the index), shell-based prompt plans (.turbo/prompt-plans/ index + separate shell files in .turbo/plans/), and regular plans without frontmatter. Use when the user asks to "migrate turboplans", "migrate turbo plans", "upgrade plan format", "add frontmatter to plans", or "convert old plans".