fix(docs): broken mermaid diagram in learn/benefits/AgentMemory.md (reserved graph keyword)

Problem

The mermaid flowchart in learn/benefits/AgentMemory.md (the "Three substrates, one memory" diagram) fails to render — the portal shows Syntax error in text / mermaid version 11.15.0.

Root cause (V-B-A'd)

The diagram declares classDef graph ... and a node Graph[...]:::graph. graph is a reserved mermaid diagram-type keyword (the alternative to flowchart), and the 11.x parser rejects it as a classDef/identifier.

Verified it is NOT the parentheses in the quoted node labels: learn/agentos/cloud-deployment/WhyDeploy.md:29 uses the identical KB["Knowledge Base (semantic RAG)"] pattern and renders fine. No other working diagram in learn/ uses a graph-named classDef.

Fix

Rename the reserved identifier: classDef graph → classDef edgegraph, node Graph → EdgeGraph, and the :::graph / edge references to match. Pure rename; diagram semantics unchanged.

Acceptance Criteria

1. The AgentMemory.md diagram renders (no mermaid syntax error) on mermaid 11.x.
2. No graph-as-identifier (classDef name or node id) remains in the diagram.