docs: add proposal for Agent document type support in MDL

Adds a comprehensive proposal for SHOW/DESCRIBE/CREATE/DROP AGENT
commands targeting the new CustomBlobDocuments$CustomBlobDocument
type used by Mendix 11.9's Agent Editor extension. Includes BSON
structure analysis, MDL syntax design, phased implementation plan,
and 7 end-to-end examples demonstrating smart app patterns: tool
microflows, MCP client/server, knowledge base RAG, conversational
UI, single-call processing, and tool approval workflows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako