Closes#8899
- schemas/prompt.json: Draft-07 JSON Schema documenting the prompt format
- Node objects with class_type (required), inputs (required), _meta (optional)
- Node links as [source_id, output_index] arrays
- Self-validating examples included in schema
- docs/api/prompt-schema.md: Documentation with validation rules, examples,
and common error types
Based on analysis of execution.py:validate_prompt() and server.py
