fix(engine-b): run guest-agent commands through /bin/sh -c

The Proxmox guest agent execs a program directly; our compound commands
(cd/&&/pipes/redirects/env) need an explicit shell.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/Services/Proxmox/HttpProxmoxClient.php

@@ -95,8 +95,11 @@ class HttpProxmoxClient implements ProxmoxClient
 
     public function guestExec(string $node, int $vmid, string $command): array
     {
-        $pid = $this->http()->asForm()
-            ->post("/nodes/{$node}/qemu/{$vmid}/agent/exec", ['command' => $command])
+        // The guest agent runs a program directly, not a shell — wrap compound
+        // commands (cd/&&/pipes/redirects/env) in an explicit shell. command is a
+        // JSON array so each argument is passed safely without re-quoting.
+        $pid = $this->http()
+            ->post("/nodes/{$node}/qemu/{$vmid}/agent/exec", ['command' => ['/bin/sh', '-c', $command]])
             ->throw()->json('data.pid');
 
         // Poll exec-status until the command has exited.