Fix diff command to use provided stage arguments in shell

The diff command was ignoring positional stage arguments (e.g., "diff staging production")
and using the current context stage instead. This made it impossible to compare
different stages in the interactive shell.

Fixed by adding special handling in KeepProxyCommand for the diff command:
- Positional arguments are now treated as stage names
- Stages are converted to --stage=stage1,stage2 format
- "unmask" is still recognized as a boolean flag

Now the shell properly supports:
- diff staging production
- diff staging production unmask
- diff local staging production

This matches the documented behavior and user expectations.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

Author: 3 people

src/Shell/Commands/KeepProxyCommand.php

@@ -77,6 +77,23 @@ protected function execute(InputInterface $input, OutputInterface $output): int
      */
     private function parseNaturalSyntax(array $args, array &$positionalArgs, array &$options): void
     {
+        // Special handling for diff command - positional args are stages
+        if ($this->keepCommand === 'diff') {
+            $stages = [];
+            foreach ($args as $arg) {
+                if ($arg === 'unmask') {
+                    $options['unmask'] = true;
+                } else {
+                    // Treat as a stage name
+                    $stages[] = $arg;
+                }
+            }
+            if (!empty($stages)) {
+                $options['stage'] = implode(',', $stages);
+            }
+            return;
+        }
+        
         // Define known option keywords for each command
         $knownOptions = [
             'show' => [
@@ -92,9 +109,6 @@ private function parseNaturalSyntax(array $args, array &$positionalArgs, array &
                 'json' => ['format', 'json'],
                 'table' => ['format', 'table'],
             ],
-            'diff' => [
-                'unmask' => ['unmask', true],
-            ],
             'export' => [
                 'json' => ['format', 'json'],
                 'env' => ['format', 'env'],