[BUG] ClaudeCodeAdd fails on paths with spaces

## Bug Description

ClaudeCodeAdd with default keybinding fails on filepaths that contain spaces

## To Reproduce

Steps to reproduce the behavior:

1. Start Claude Code integration with a path containing spaces
2. Run command 'ClaudeCodeAdd %'
3. See error Too many arguments

## Expected Behavior

Default command should handle paths with spaces.

## Fix:
Update readme:

```
{
"ab",
function()
require("claudecode").send_at_mention(vim.fn.expand("%:p"), nil, nil, "ClaudeCodeAdd")
end,
desc = "Add current buffer",
}
```

Update the README's default ClaudeCodeAdd keybinding using the Lua example in the issue. Check the documented command with a current buffer whose path contains spaces and confirm it no longer produces the "Too many arguments" error.

Written by the indexing model from the issue text.