feat(draft-docs): add Tutorial content type support

[hey-amelia]

Summary

Add support for generating Tutorial-style documentation to the /beagle:draft-docs command.

Context

The draft-docs command currently supports Reference and How-To content types. Tutorials are learning-oriented documentation that help beginners through step-by-step practice.

Requirements

• Create skills/mintlify-tutorial-docs/SKILL.md with tutorial-specific guidance
• Update draft-docs.md command to detect/offer Tutorial type
• Tutorial characteristics (from Diátaxis):
  • Assumes no prior experience
  • Sequential, linear progression
  • Set clear expectations about outcomes
  • Incremental steps with minimal choices
  • Highlight milestones throughout
  • Emphasize actions over theory

Related

• Design doc: docs/plans/2025-12-25-draft-docs-command-design.md
• Parent issue: Replace VitePress docs with Mintlify for v1.0.0 release amelia#142