Support for Custom Commands

[intellectronica]

Support installing custom commands in agents that support them.

• Copilot: https://code.visualstudio.com/docs/copilot/customization/prompt-files
• Codex CLI: https://github.com/openai/codex/blob/main/docs/prompts.md
• Claude Code: https://docs.anthropic.com/en/docs/claude-code/slash-commands#custom-slash-commands
• opencode: https://opencode.ai/docs/commands/
• Cursor: https://docs.cursor.com/en/agent/chat/commands
• Windsurf: https://docs.windsurf.com/windsurf/cascade/workflows
• RooCode: https://docs.roocode.com/features/slash-commands

For other agents, we can try to fake commands by adding explicit instructions to the rulers (like If the user sends "/code-review" do the following....)