Add 'How to Add an Agent' developer guide

[mingshl]

Originally created by @mingshl in mingshl/opensearch-agent-server#22

Description

Create a developer guide documenting how to add a new sub-agent to the server. This is critical for the ART team and any future contributors.

Content outline

1. Create agent factory in src/agents/my_agent.py
2. Define system prompt and tool selection
3. Register in ag_ui_app.py lifespan
4. Configure page context routing
5. (Optional) Add custom MCP tool profile
6. Add tests
7. End-to-end verification

Include

• Code examples for each step
• Template agent file that can be copied
• Explanation of the factory pattern and how orchestration works
• How to test with a mock OpenSearch

Location

docs/adding-agents.md (linked from CONTRIBUTING.md)

References

• Existing agents: src/agents/fallback_agent.py, src/agents/art_agent.py
• Registration: src/server/ag_ui_app.py