[feature] AI Copilot for Evaluations

## Summary

Add AI Copilot integration for Agent Evaluations — enabling AI-assisted test scenario creation, judge configuration, and evaluation analysis through the CopilotPanel.

## Features

### 1. AI-Assisted Test Scenario Creation

The CopilotPanel can interactively guide the user through creating evaluation test scenarios:

- User describes the behavior they want to test in natural language
- Copilot asks clarifying questions (expected outputs, edge cases, multi-turn vs single-turn)
- Copilot generates well-structured test scenarios with appropriate user messages and expected behaviors
- User can iterate on the generated scenarios before saving
- Copilot can suggest additional scenarios to improve coverage

### 2. AI-Assisted Judge Configuration

When setting up judges for evaluations:

- Copilot can recommend appropriate judge types (LLM rule-based vs deterministic) based on what the user wants to evaluate
- Copilot generates judge rules and criteria from natural language descriptions
- User can ask Copilot to create matching judge configurations for existing test scenarios
- Copilot understands available judge types (contains text, regex, response length, JSON schema, similarity, LLM rule-based) and suggests the best fit

### 3. Evaluation Results Analysis

After evaluation runs complete:

- Copilot can analyze evaluation results and explain failures
- User can ask Copilot why specific scenarios failed and get actionable improvement suggestions
- Copilot can compare results across runs and identify regressions
- Copilot can suggest agent prompt improvements based on evaluation patterns

## Context

This builds on the Agent Evaluations feature (#4553) which provides the evaluation infrastructure (test scenarios, judges, async runs, results tracking). The Copilot integration uses the existing `CopilotPanel` infrastructure and `useCopilotStore`.

## Acceptance Criteria

- [ ] CopilotPanel can generate test scenarios from natural language descriptions
- [ ] Copilot can recommend and configure appropriate judges for evaluation goals
- [ ] Generated scenarios include proper structure for both single-turn and multi-turn conversations
- [ ] Users can iterate on generated scenarios and judges before saving
- [ ] Copilot can analyze evaluation run results and suggest improvements
- [ ] Copilot context includes current evaluation configuration when editing