feat(ai): add AI-powered commit message generation

[CalvinAllen]

Description

Add AI-powered features to assist with commit message generation, change explanations, and other git-related text generation tasks.

Features

AI Commit Message Generation

• Generate commit messages from staged changes
• Analyze diffs to understand change context
• Follow conventional commit format (optional)
• Customizable prompt templates
• Edit generated message before committing

AI Stash Message Generation

• Generate descriptive stash messages
• Summarize working changes intelligently

AI Change Explanation

• Explain what a commit does in plain language
• Explain branch changes (all commits)
• Explain working directory changes
• Explain stash contents

AI Changelog Generation

• Generate changelog from selected commits
• Group changes by type (features, fixes, etc.)
• Customizable output format

AI Provider Options

• GitHub Copilot (if installed)
• OpenAI API (bring your own key)
• Azure OpenAI
• Anthropic Claude API
• Local models (Ollama)
• Custom API endpoints

Configuration Options

• Default AI provider selection
• API key management (secure storage)
• Prompt customization
• Conventional commit enforcement
• Maximum token/length limits
• Temperature/creativity settings

UI Integration

• Generate button in commit dialog
• Context menu in commit views
• Command palette commands
• Keyboard shortcuts

Acceptance Criteria

• Commit message generation produces relevant messages
• Multiple AI providers can be configured
• API keys are stored securely
• Generated text can be edited before use
• Explanations are accurate and helpful
• Feature gracefully handles API errors