feat: SSH terminal - the agent connects by SSH to another host (password, key file or its own keys) and the shell opens in a tab with the connection's progress shown; host keys trusted on first use, a changed one refused

internal/proto/proto.go

@@ -18,8 +18,23 @@ const (
 	CtrlOpenForward = "open_forward"
 	// CtrlTask tells the agent to fetch and run the one-shot task TaskID.
 	CtrlTask = "task"
+	// CtrlOpenSSH tells the agent to open an SSH connection to SSH and bridge
+	// its shell to the session id, as a terminal.
+	CtrlOpenSSH = "open_ssh"
 )
 
+// SSHTarget is where an SSH terminal connects and how it signs in: a
+// password, a private key (PEM or OpenSSH, with its passphrase if it has
+// one), or, with neither, the agent user's own keys in ~/.ssh.
+type SSHTarget struct {
+	Host       string `json:"host"`
+	Port       int    `json:"port,omitempty"`
+	User       string `json:"user"`
+	Password   string `json:"password,omitempty"`
+	Key        string `json:"key,omitempty"`
+	Passphrase string `json:"passphrase,omitempty"`
+}
+
 // EnrollRequest is sent once by a freshly installed agent.
 type EnrollRequest struct {
 	Name     string `json:"name"`
@@ -38,11 +53,12 @@ type EnrollResponse struct {
 
 // ControlMessage is the body returned by the control long-poll.
 type ControlMessage struct {
-	Type      string `json:"type"`
-	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)
+	Type      string     `json:"type"`
+	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)
+	SSH       *SSHTarget `json:"ssh,omitempty"`     // CtrlOpenSSH
 }
 
 // Transports an agent can reach the server by.
@@ -96,8 +112,20 @@ type InBatch struct {
 type OutMessage struct {
 	Data string `json:"data,omitempty"` // base64 bytes
 	Exit bool   `json:"exit,omitempty"`
+	// Status tells the panel how an SSH terminal's connection goes: one of
+	// the SSHStatus values, with Note to show.
+	Status string `json:"status,omitempty"`
+	Note   string `json:"note,omitempty"`
 }
 
+// The steps of an SSH terminal's connection, as OutMessage.Status.
+const (
+	SSHDial = "dial" // connecting to the host
+	SSHAuth = "auth" // the host answered, signing in
+	SSHOK   = "ok"   // signed in, the shell runs
+	SSHErr  = "err"  // it failed; Note says why
+)
+
 // --- one-shot tasks (file manager, native in the agent) ---
 
 const (