Support For dynamic File References in Flow Config

The nodes are embedded inline with formatting artifacts (hello\\nhello), apparent test content, and no versioning metadata. Editing it requires understanding the entire config file, and changes produce noisy diffs that obscure actual prompt improvements.

Before:

prompts: 
- id: 187c2f4b-c23d-4545-abef-73dc897d6b7b 
role: assistant 
content: >- hello hello Important:please provide the response in wrap text format... 

After:

prompts: 
- id: rag_system_prompt_v1 
role: assistant # Edit prompts in /prompts/rag_system.md — supports preview & versioning 
contentRef: ./prompts/rag_system.md 
version: "1.2.0" 
lastUpdated: "2025-01-15" 

Developer Impact: Prompts become first-class artifacts — reviewable in PRs, A/B testable, and editable without touching pipeline config.