Automatically cd into worktree directory after creation

## Summary
Currently, when creating a new worktree or when an existing worktree is found, quicktree prints a `cd` command that the user needs to manually execute. This should be improved to automatically change the current shell's directory to the worktree.

## Current behavior
- When creating a new worktree: prints `cd /path/to/worktree` (line 218)
- When finding an existing worktree: prints `cd /path/to/worktree` (line 201)

## Proposed behavior
Automatically change the current shell's working directory to the worktree path after creation or when an existing worktree is found.

## Implementation considerations
Since quicktree is a Node.js script running as a subprocess, it cannot directly change the parent shell's directory. Possible solutions:
1. Use a shell function wrapper that sources quicktree output
2. Output shell commands that can be evaluated by the parent shell
3. Integration with shell configuration files

## Test plan
- [ ] Test creating a new worktree - should automatically cd into it
- [ ] Test when an existing worktree is found - should automatically cd into it
- [ ] Test with different shells (bash, zsh, fish)
- [ ] Ensure backward compatibility for users who rely on the current behavior