Support for Modes / Primary Agents

[intellectronica]

Support installing custom modes (sometimes called primary agents) in agents that support them.

• opencode: https://opencode.ai/docs/agents/#primary-agents
• Copilot: https://code.visualstudio.com/docs/copilot/customization/custom-chat-modes#_custom-chat-modes
• Claude Code: https://docs.anthropic.com/en/docs/claude-code/output-styles
• Cursor: https://docs.cursor.com/en/agent/modes#custom