[FEATURE] Horizontal terminal split

## Feature Description

Opening the Claude Code in a horizontally split terminal window.

## Use Case

Screen real estate is really preferential, so it's a good idea to support as many configurations as possible.

## Proposed Solution

`split_side` should take a 3rd option: `"bottom"` (maybe include `"top"` as 4th?), which then splits the terminal horizontally. `split_width_percentage` could be just `split_percentage` or `split_ratio`.

## Alternatives Considered

It's possible to just rotate the window after creation (screenshot included), but it would be nice to skip that manual step by making it configurable.

## Additional Context

Start by locating the split_side and split_width_percentage configuration and the terminal-window creation entry point in the Neovim plugin. Trace how the existing side split is applied, then verify that a bottom or top option and a split percentage produce the requested horizontal layout.

Written by the indexing model from the issue text.