Improve youmind SKILL.md with clearer capability index and usage guidance

[cellinlab]

Summary

This PR improves skills/youmind/SKILL.md to make the official YouMind CLI skill easier to discover and use in agent environments like OpenClaw.

What changed

• rewrote the frontmatter description to improve triggering coverage
• clarified the recommended search -> info -> call workflow
• added a functional index for common task areas:
  • boards
  • notes
  • materials
  • crafts
  • picks
  • documents
  • slides
  • chats
  • skills
  • scheduled tasks
• added more direct examples for common operations
• documented currently verified APIs:
  • createBoard
  • getBoard
  • createNote
  • createMaterialByUrl
  • listMaterials
• documented an important capability boundary:
  • local file upload does not appear to be exposed in the current official CLI surface

Why

The original skill is correct but fairly minimal. In practice, agent runtimes benefit a lot from:

• richer trigger language
• clearer task-to-command mapping
• explicit examples for common workflows
• documented boundaries around what is and is not currently available

This makes the skill easier to trigger automatically and easier to use directly without trial-and-error.

Validation

Validated locally with the official CLI:

• youmind search
• youmind info
• youmind call createBoard
• youmind call getBoard
• youmind call createNote
• youmind call createMaterialByUrl
• youmind call listMaterials