refactor: restructure microagents system

[xingyaoww]

This PR restructures the microagents system to provide a clearer separation between repository-specific instructions and shareable microagents.

Key Changes

1. Two Sources for Instructions

   • Repository Instructions: .openhands/microagents/repo.md in each repository
     • Private to each repository
     • Repository-specific guidelines
     • Team practices and conventions
   • Shareable Microagents: OpenHands/microagents/
     • Available to all users
     • Reusable knowledge and workflows
     • Maintained in OpenHands repository

2. Simplified Hub Structure

   OpenHands/microagents/
   ├── knowledge/  # Keyword-triggered expertise
   └── tasks/     # Interactive workflows
   

3. Documentation

   • Clear separation between private and shareable content
   • Better examples and guidelines
   • Clearer contribution process
   • Loading order explanation

4. Code Changes

   • New microagent models with validation
   • Support for both knowledge and task agents
   • Improved loading and matching system
   • Comprehensive test coverage

Migration Notes

• Move repository-specific instructions to .openhands/microagents/repo.md in each repository
• Convert shareable agents to new YAML format
• Keep private content in repositories, share reusable knowledge in OpenHands

Part of #5538 and #5826

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:226ae37-nikolaik   --name openhands-app-226ae37   docker.all-hands.dev/all-hands-ai/openhands:226ae37