feat(commands): add AI git commit message generator

Add comprehensive slash command for generating conventional commit
messages using AI. The command analyzes staged git changes and creates
well-structured commit messages following best practices.

Features:
- Conventional commit format (type, scope, description)
- Interactive message editing and regeneration
- Context-aware analysis of git diffs
- Learning from existing commit history style
- Support for custom type and scope hints
- Multi-step workflow with user confirmation

Command supports multiple usage patterns:
- Basic: /git-commit-msg
- With hints: /git-commit-msg fix
- With context: /git-commit-msg "feat(api)"

Also updates command README with documentation for the new command.

🤖 Generated with Claude Code

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