Add an option to automatically create a git commit when a task is marked as done.

## Requirements

1. Add a new setting `auto_commit_on_done` to `config.toml`
   - Default value: `false` (off by default)
   - When enabled, automatically create a git commit when `set_task_status` is called with `status="done"`

2. The commit should:
   - Include all staged and unstaged changes
   - Use a meaningful commit message that references the task name
   - Example message format: "Complete task: {task-name}" or similar

3. Update CLAUDE.md to document this new setting

## Behavior

- If `auto_commit_on_done = false` (default): No automatic commit, current behavior
- If `auto_commit_on_done = true`: When a task transitions to "done" status, automatically run git add and git commit