Report a failed spawn, drop the paste wrapper and the unbound close command, sync docs

- vterm_open shows an error dialog when the command cannot start,
  instead of a tab that silently vanishes
- on_text_command hands paste straight to vterm_paste_text;
  VtermPasteCommand only forwarded
- VtermCloseCommand was bound nowhere; closing the tab already kills
  the process
- wheel_lines fallback matches the shipped 1; README said three
- Architecture.md: per-theme scheme files and managers, command table

main.py

@@ -12,14 +12,12 @@ from .vterm.commands import (
     VtermClearScreenCommand,
     VtermClearUndoStackCommand,
     VtermCleanupCommand,
-    VtermCloseCommand,
     VtermCopyCommand,
     VtermInitializeViewCommand,
     VtermKeypressCommand,
     VtermNoopCommand,
     VtermOpenCommand,
     VtermOpenTerminalProjectFolderCommand,
-    VtermPasteCommand,
     VtermPasteSelectionCommand,
     VtermPasteTextCommand,
     VtermSelectThemeCommand,