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/proto/proto.go

@@ -27,6 +27,7 @@ type EnrollRequest struct {
 	OS       string `json:"os"`
 	Arch     string `json:"arch"`
 	Version  string `json:"version"`
+	User     string `json:"user,omitempty"` // OS user the agent runs as
 }
 
 // EnrollResponse carries the identity the agent persists and uses from then on.
@@ -41,6 +42,7 @@ type ControlMessage struct {
 	SessionID string `json:"session_id,omitempty"`
 	Port      int    `json:"port,omitempty"`    // CtrlOpenForward: client-side port to dial
 	TaskID    string `json:"task_id,omitempty"` // CtrlTask
+	User      string `json:"user,omitempty"`    // CtrlOpenTerminal: OS user for the shell ("" = the agent's own)
 }
 
 // Terminal / stream frame types on the session "in" channel (server -> agent).
@@ -84,7 +86,9 @@ const (
 	TaskFSMove     = "fs.move"     // move/rename
 	TaskFSDownload = "fs.download" // read a whole file (larger cap) for browser download
 
-	TaskSysUpdate = "sys.update" // download the latest agent binary and restart
+	TaskSysUpdate  = "sys.update"  // download the latest agent binary and restart
+	TaskSysUsers   = "sys.users"   // report the agent's user and the login-capable users
+	TaskSysUserAdd = "sys.useradd" // create a new OS user
 )
 
 // TaskEnvelope is what the agent fetches for a task id.
@@ -144,3 +148,17 @@ type FSReadResult struct {
 	Truncated bool   `json:"truncated"`
 	Binary    bool   `json:"binary"`
 }
+
+// SysUsersResult is returned for TaskSysUsers.
+type SysUsersResult struct {
+	Current   string   `json:"current"`    // user the agent runs as
+	CanSwitch bool     `json:"can_switch"` // agent is root, so it can open shells as (and create) other users
+	Users     []string `json:"users"`
+}
+
+// SysUserAddArgs are the arguments of TaskSysUserAdd.
+type SysUserAddArgs struct {
+	Name     string `json:"name"`
+	Password string `json:"password,omitempty"` // empty leaves the account without a password
+	Sudo     bool   `json:"sudo"`               // add to the sudo/wheel group
+}