Fixed 2 issues and added 5 features

[fengwk]

Fix

1. Calling Signs.set_for_lines with a busy call will cause neovim to freeze.
2. Resolved chat window unmodifiable error and avoid streaming response clearing on panel redraw.

Features

1. Support setting a separate close window keymap in normal mode.
2. Support opening the chat window with options, which is useful when carrying selected content into the chat window.
3. Allow ignoring the default action path, as the default actions.json specifies models. This option provides a way for users to customize actions with the same content when those specified models are not needed.
4. Support a default system message.