feat: terminal as a chosen OS user, create users from the panel (0.0.2)

- agent reports the OS user it runs as (enroll + poll header)
- per-machine terminal user on the edit page; defaults to the agent's own user (root on Linux)
- create a new user from the panel: name, password, optional sudo group
- show the reason in the terminal when a shell fails to start
- bump version to 0.0.2

internal/agent/terminal.go

@@ -1,6 +1,7 @@
 package agent
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
 
@@ -13,14 +14,30 @@ type PTY struct {
 	f   *os.File
 }
 
-// StartPTY launches the shell attached to a new PTY. The agent runs as root
-// (systemd User=root), so this is a root shell by design.
-func StartPTY() (*PTY, error) {
-	shell := "/bin/bash"
-	if _, err := os.Stat(shell); err != nil {
-		shell = "/bin/sh"
+// StartPTY launches a login shell attached to a new PTY. With user empty (or
+// equal to the agent's own user) it is the agent's shell — root by default,
+// since the agent runs as a root systemd service. Any other user is entered via
+// `su -l`, which only a root agent can do without a password.
+func StartPTY(user string) (*PTY, error) {
+	var cmd *exec.Cmd
+	if user == "" || user == currentUser() {
+		cmd = exec.Command(defaultShell(), "-l")
+	} else {
+		if !validUserName(user) {
+			return nil, fmt.Errorf("недопустимое имя пользователя %q", user)
+		}
+		if os.Geteuid() != 0 {
+			return nil, fmt.Errorf("агент запущен от %s и не может открыть сессию от %s", currentUser(), user)
+		}
+		if !userExists(user) {
+			return nil, fmt.Errorf("пользователь %s не найден", user)
+		}
+		su, err := exec.LookPath("su")
+		if err != nil {
+			return nil, fmt.Errorf("su не найден: %w", err)
+		}
+		cmd = exec.Command(su, "-l", user)
 	}
-	cmd := exec.Command(shell, "-l")
 	cmd.Env = append(os.Environ(), "TERM=xterm-256color")
 	f, err := pty.Start(cmd)
 	if err != nil {