feat: add planning and realization stage prompt assets

[saschagobel]

Summary

Introduce canonical system and user prompt assets for the synthetic query generation workflow. This defines stable stage-level instructions and invocation templates for both planning and realization, aligned with the two-stage query generation design.

Key changes

• Add SYSTEM_PROMPT_PLANNING and USER_PROMPT_PLANNING
  • Weighted sampling over query-generation specification
  • Explicit dimension definitions, constraints, and distribution-aware behavior
• Add SYSTEM_PROMPT_REALIZATION and USER_PROMPT_REALIZATION
  • One-to-one realization of blueprints into natural-language user queries
  • Clear separation of task construction vs behavioral guardrails
• Establish consistent prompt structure across stages:
  • ROLE, INPUT, OUTPUT, TASK, CONSTRAINTS
• Enforce structured output discipline via schema-aligned instructions
• Keep user prompts lean with runtime placeholders and model-legible payloads

Status

Ready for review.

Closes #58