Make system prompt dynamic based on mode by using MCP Prompts

[IntranetFactory]

The system prompt contains instructions which might not be needed for each mode.

MCP creation #558 or attempt_completion #406 could become a MCP server which could be enabled per mode with #639

https://modelcontextprotocol.io/docs/concepts/prompts indicates that MCP could return Prompts. So for example when the MCP creation becomes a MCP server it could also return the prompt to be inserted when it's enabled in a specific mode.