feat(skill): add agent-spec-designer for file-based agent design

[Baduc81]

Summary

This PR introduces a new OpenHands Skill: agent-spec-designer.

The skill enables users to create high-quality, Markdown-based sub-agents through an interview-style workflow, instead of manually writing specifications.

Motivation

Currently, creating file-based agents requires users to:

• Understand the Markdown schema
• Manually define all requirements
• Handle edge cases themselves

This creates friction, especially for new users.

This skill addresses the problem by guiding users step-by-step and ensuring complete, structured agent specifications.

Key Features

• Interview-style requirement gathering
• One-question-at-a-time interaction (reduces cognitive load)
• Mandatory requirement validation:
  • Goal
  • Tasks
  • Input / Output
  • Constraints
  • Edge Cases
  • Success Criteria
  • And other aspects depend on the situation
• Detection of missing or ambiguous requirements
• Smart follow-up questions
• Suggestion of additional design dimensions (tools, execution, validation, etc.)
• Generation of clean, implementation-ready Markdown specs

Example Workflow

1. User triggers:
   /agent-designer

2. Agent asks structured questions

3. Requirements are refined interactively

4. Final agent spec is generated

Impact

• Improves onboarding experience for new users
• Increases quality and consistency of generated agents
• Reduces manual effort and errors
• Encourages best practices in agent design

Files Added

• SKILL.md
• README.md

Notes for Reviewers

• The skill enforces strict requirement completeness before generation
• Designed to be adaptive to user expertise level
• Focused on real-world usability rather than theoretical completeness