Load user plugins after built-in plugins

Improvements

• CoreFix unhandled errors when aborting with queued messages
• CoreUser plugins override built-in plugins for the same provider@rmk40
• CoreMove Codex 5.3 model definition to plugin to avoid showing unsupported model to other users
• CoreAdd session usage tracking to ACP@SteffenDE
• CoreUpdate transforms for GPT-5.3
• TerminalAllow mouse escape via "esc" labels in dialogs@AksharP5
• DesktopMake close comment button visible in prompt input@alexyaroshuk
• DesktopHide prompt input when there are permissions requests or questions
• DesktopMore terminal stability fixes
• DesktopModified file color contrast in app for better visibility
• DesktopAdd button to open files in external applications
• DesktopAllow toggling file tree closed independently
• DesktopStop showing SessionSkeleton on new workspace@dbpolito
• DesktopSet workspace name earlier to improve creation and deletion@dbpolito