[FEATURE] Implement Git commands (cx clone, cx push, cx pr)

[mikejmorgan-ai]

Description

Git integration commands mentioned in roadmap are not implemented.

Required Commands

1. cx clone <repo> - Clone repository with AI assistance
2. cx push - Commit and push with auto-generated message
3. cx pr <title> - Create pull request

Current State

• Status bar shows git branch (working)
• No CLI commands for git operations

Implementation

1. Add git module to CLI
2. Implement clone with progress tracking
3. Implement push with AI commit message generation
4. Implement PR creation via gh CLI or GitHub API

Estimated Time

4+ hours