Feature Request: Add Vim-style keyboard navigation (Ctrl+J/K) to file picker

## Description
When using the @ symbol to select files in Claude Code, I would like to navigate the file list using Vim-style keyboard shortcuts, specifically:

• Ctrl+J to move down
• Ctrl+K to move up

This is similar to how fzf and other terminal tools work with Vim keybindings enabled.

## Current Behavior
Currently, file navigation in the file picker only works with arrow keys (up/down).

## Expected Behavior
When the file picker is open (after typing @), users should be able to:

• Press Ctrl+J to move the selection down
• Press Ctrl+K to move the selection up
• Optionally: Ctrl+N for down and Ctrl+P for up (common Vim alternatives)

## Use Case
Many developers who use terminal tools like fzf, telescope (Neovim), or other fuzzy finders are accustomed to using Ctrl+J/K for navigation. This would make Claude Code's
file picker feel more familiar and improve workflow efficiency for Vim users.

## Additional Context

• Claude Code already has a /vim mode for the main editor
• This feature would specifically enhance the file picker experience
• Similar navigation is standard in many terminal-based fuzzy finders

## Possible Implementation
Could be implemented as:

1. A default behavior when /vim mode is enabled
2. A separate configuration option for file picker keybindings
3. Always-on feature for all users