fix(cli): Prevent truncation of first character in shell commands

- The shell command processor was incorrectly truncating the first
  character of the command (e.g., 'ls' became 's') due to an
  erroneous `slice(1)` operation, likely introduced during a
  previous merge. This change removes the slice, ensuring the full
  command is processed.
- Introduces unit tests for the shellCommandProcessor hook.
- Fixes a minor grammatical issue in the display of GEMINI.md file count.

packages/cli/src/ui/App.tsx

@@ -348,7 +348,8 @@ export const App = ({
                     <Text color={Colors.AccentRed}>|⌐■_■| </Text>
                   )}
                   <Text color={Colors.SubtleComment}>
-                    Using {geminiMdFileCount} GEMINI.md files
+                    Using {geminiMdFileCount} GEMINI.md file
+                    {geminiMdFileCount > 1 ? 's' : ''}
                   </Text>
                 </Box>
               )}