fix(P20): system user consistency (REQ-135, F23)

---ci---
project: orca
phase: 20
milestone: v0.12
status: execute
---/ci---

Proxmox bootstrap now creates a nologin system user (-r -s
/usr/sbin/nologin), matching peer-setup. Previously it created a
login user (-m -s /bin/bash) with more privilege. Build + tests green.

internal/proxmox/bootstrap.go

@@ -392,7 +392,7 @@ func deployPubKey(user, pubLine string) error {
 // createLinuxUser creates the orca system user if it doesn't already
 // exist. Idempotent: `id -u` check before `useradd`.
 func createLinuxUser(user string) error {
-	cmd := fmt.Sprintf("id -u %s 2>/dev/null || useradd -m -s /bin/bash %s", user, user)
+	cmd := fmt.Sprintf("id -u %s 2>/dev/null || useradd -r -s /usr/sbin/nologin %s", user, user)
 	if _, err := runRemote(cmd); err != nil {
 		return err
 	}