[Orchestra] Agents missing system prompts

Problem

All agents (Planner, Plan Reviewer, Engineer, Code Reviewer) have system_prompt: null. Without system prompts, the agents don't know their specific roles.

Expected Behavior

Each agent should have a system prompt that defines:

• Their role and responsibilities
• What kind of output they should produce
• How they should interact with the codebase

Current Agent States

Planner: system_prompt: null
Plan Reviewer: system_prompt: null
Engineer: system_prompt: null
Code Reviewer: system_prompt: null

Impact

Without system prompts:

• Planner analyzed existing code instead of creating an implementation plan
• Agents don't understand their specific workflow roles
• The multi-agent workflow cannot function as designed

Proposed Solution

1. Allow setting system prompts for agents via UI or API
2. Provide default system prompts for common agent roles
3. Consider using the instructions_content or instructions_path fields that exist in the Agent model