Migrate Turbo Files

Current layouts:

• Unexpanded shells live in .turbo/shells/ with spec: and depends_on: frontmatter.
• Plans live in .turbo/plans/ with status: (required) and optional spec: for provenance.
• .turbo/improvements.md entries use Type values direct, investigate, or plan. Legacy values trivial and standard map to direct and plan respectively.

This skill migrates legacy shapes to those layouts.

Task Tracking

Use TaskCreate to create a task for each step: