docs: update enterprise features and add Flow Builder specifications

Update the enterprise functionality documentation to include the new Flow Builder (MVP) visual canvas, featuring n8n-style blocks, draft/versioning systems, and runtime validation.

Additional updates include:
- Enhanced bot handover logic (expiry handover and unassigned queue).
- Improved AI Assistant capabilities including trusted citations and external bot exclusivity.
- Expanded MCP (Model Context Protocol) integration with typed contracts and authenticated execution.