[Feature Request] Add config option to disable copy-on-select behavior

OC implements "XTerm / GPM"-style copy-on-select

Selecting text with the mouse automatically copies it to the clipboard when the mouse button is released. As far as I can find, there is no way to disable this behavior.

I'd like to be able to turn off copy-on-select via opencode.json config because frankly it interferes with the way I read and mark text on screen, in particular when screen sharing or pair coding. And I really don't appreciate accidental clipboard overwrites when scrolling/selecting to read text.

I understand that the TUI is mouse-aware, and I am aware that in e.g. Ghostty's config I can set mouse-shift-capture = never to make shift+click always selects text, without notifying the app, but I'd still like to prevent OC from bungling my current clipboard contents.

Proposed Solution

I guess a tui.copy_on_select config option would suffice. Keep its default as true to avoid surprising existing users.

{
  "tui": {
    "copy_on_select": false
  }
}

Current Behavior

Mouse selection → automatic clipboard copy

Expected Behavior (when disabled)

Allows users to select text visually without side effects