feat: add show_terminal_on_at_mention configuration option

[John-Lin]

Summary

This PR adds a new configuration option show_terminal_on_at_mention to allow users to disable automatic terminal visibility when sending @ mentions to Claude.

Motivation

Some users prefer to manage terminal sessions externally using tools like Ghostty, Tmux, or other terminal multiplexers. For these workflows, automatically opening/showing the terminal split in Neovim when sending @ mentions can be disruptive to their preferred window management setup.

Changes

• New config option: show_terminal_on_at_mention (boolean, defaults to true)
• Conditional terminal display: Terminal is only shown/opened when the option is enabled
• Backward compatibility: Existing behavior is preserved by default
• Config validation: Added validation for the new boolean option

[John-Lin]

Checkout to feature branch and fix test in #90