[feature] AI Copilot for Skills

## Summary

Add AI Copilot integration for Agent Skills — enabling AI-assisted skill creation, editing, and SKILL.md content generation through the CopilotPanel.

## Features

### 1. Create from AI (CopilotPanel-powered)

The existing "Create with AI" option in the skills panel should use the CopilotPanel to interactively guide the user through skill creation:

- User describes the desired skill behavior in natural language
- Copilot asks clarifying questions (scope, triggers, edge cases)
- Copilot generates well-structured SKILL.md content following the [Agent Skills specification](https://agentskills.io/specification) — proper YAML frontmatter (name, description), clear step-by-step instructions, decision logic, examples, and reference sections or script stubs when appropriate
- User can iterate on the generated content before saving
- Copilot packages the result as a `.skill` archive and creates it via the existing `createAgentSkillFromInstructions` API

### 2. AI-Assisted Skill Editing

When viewing an existing skill in the detail view:

- Copilot can suggest improvements to SKILL.md instructions
- User can ask Copilot to refine, expand, or restructure skill content
- Copilot understands the skill's current content and can make targeted edits

## Context

This builds on the Agent Skills feature (#4545) which provides CRUD operations for skills. The Copilot integration uses the existing `CopilotPanel` infrastructure and `useCopilotStore`.

## Acceptance Criteria

- [ ] "Create with AI" option opens CopilotPanel with skill creation context
- [ ] Copilot can generate complete SKILL.md from natural language description following the Agent Skills specification
- [ ] Generated skills include proper YAML frontmatter, structured instructions, and optional reference/script sections
- [ ] Users can iterate on generated content before saving
- [ ] Copilot context includes current skill content when editing existing skills