fix: escape {{template variables}} in assemble to prevent DSH prompt interpolation errors

[Liyuk]

Problem

Node content may contain {{unsubscribe_url}} or similar template syntax (e.g. from Buttondown email templates). When graph-memory recalls these nodes and injects them into the graph-memory:recall context, DSH's prompt template system scans for {{variable}} patterns and throws:

unknown prompt variable "{{unsubscribe_url}}" in context "graph-memory:recall"

This affects any node whose content happens to contain double-curly-brace syntax.

Fix

Escape {{ and }} as HTML entities ({{ / }}) in the escapeXml function in assemble.ts. This prevents DSH's interpolation from interpreting them as template variables.

• Only paired braces ({{ / }}) are escaped — single { or } are preserved for CSS/JSON compatibility
• All content passing through escapeXml (node body, description, edge conditions, episodic messages) is protected

Example

Before: {{unsubscribe_url}} → DSH tries to resolve → throws error
After: {{unsubscribe_url}} → passes through literally ✅