docs: update agent proposal with real BSON samples for all 4 doc types

The test3 project now contains examples of all four agent-editor
document types (Agent, Model, Knowledge Base, Consumed MCP Service)
plus a fully-populated Agent with MCP and KB references. This replaces
earlier speculation with observed schemas.

Key updates:
- BSON Structure section now documents Contents JSON schemas for all
  four types (Model, Knowledge Base, Consumed MCP Service, Agent)
  based on real samples from test3
- Added observed values: provider=MxCloudGenAI, protocolVersion values,
  toolChoice=Auto (capitalized), tool id UUIDs, toolType=MCP/empty
- CREATE AGENT now includes Model, MaxTokens, ToolChoice as top-level
  properties (matches JSON)
- Removed Access: property from TOOL blocks — UserAccessApproval is a
  runtime-only field on AgentCommons.Tool, not in the agent document
- MCP SERVICE block uses Enabled/Description (not Access)
- KNOWLEDGE BASE block has Source:/Collection:/MaxResults:/Description:
  (dropped MinSimilarity — not in observed JSON)
- Phase 4 CREATE MODEL/KB/CONSUMED MCP SERVICE syntax revised to match
  real BSON: Provider+Key for Model/KB; ProtocolVersion+Version+
  ConnectionTimeoutSeconds+Documentation for MCP
- Added Go type definitions for Model, KnowledgeBase,
  ConsumedMCPService structs
- Parser dispatches by CustomDocumentType for all 4 types
- Added per-type catalog tables (MODELS, KNOWLEDGE_BASES, etc.)
- Resolved Open Questions 1, 2 (partial), 3, 6
- Added new Open Questions 7 (UUID stability) and 8 (Portal-populated
  fields)
- Risk Assessment updated with cross-document UUID and Portal-field
  risks

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

Author: ako