Native AI capabilities: assistant, brief generation, ChatGPT/MCP integrations

Bring AI/LLM capability into ManyRequests. Asks include: a built-in AI assistant that can create projects, update tasks, and generate reports from natural language; on-the-fly AI brief creation with brand-specific prompts; ChatGPT integration for automatic replies on requests; and an MCP server so external LLM clients (Claude, Gemini) can connect to ManyRequests data alongside Notion/Webflow/Slack.