IAV-252/feat: Add CRUD operations for Agent Custom Instructions management

Overview

This MR introduces a comprehensive custom instructions management system for agents, allowing fine-grained control over agent behavior through configurable suggestions and restrictions.

Features

• Create Instructions: Add new custom instructions to agents with suggestions/restrictions
• Update Instructions: Modify existing instruction content and settings
• Delete Instructions: Remove instructions from agents
• Enable/Disable: Toggle instructions on/off without deletion
• Retrieval: Fetch all instructions for an agent or query specific instructions by UID

Testing

• Unit tests for all CRUD operations
• Integration tests for enable/disable functionality
• Validation for UID uniqueness and agent association

Closes IAV-252