agent: add SaveSession to Core interface for external persistence

- Add Backend/Model fields to PersistedSession for full restore metadata
- Add saveToFull() that includes backend/model in persisted state
- Add exported LoadPersistedSession(path) helper
- Add SaveSession(path) to Core interface, implemented on agent struct
- Existing saveTo() preserved as backward-compatible wrapper

pkg/agent/core.go

@@ -709,6 +709,18 @@ func (s *agent) saveSession() {
 	}
 }
 
+// SaveSession writes the current session state to the given path, including
+// backend and model metadata for external restore.
+func (s *agent) SaveSession(path string) error {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	if s.session == nil {
+		return fmt.Errorf("no active session")
+	}
+	return s.session.saveToFull(path, s.sessionID, s.agentName,
+		s.runtime.Backend.Name(), s.runtime.Backend.Model(), s.CWD())
+}
+
 func (s *agent) getActionCancel() context.CancelCauseFunc {
 	if a := s.currentAction.Load(); a != nil {
 		return a.cancel

pkg/agent/session.go

@@ -37,6 +37,8 @@ Be concise. Capture what another LLM needs to seamlessly continue.`
 type PersistedSession struct {
 	ID        string            `json:"id"`
 	Agent     string            `json:"agent,omitempty"`
+	Backend   string            `json:"backend,omitempty"`
+	Model     string            `json:"model,omitempty"`
 	CWD       string            `json:"cwd,omitempty"`
 	Messages  []backend.Message `json:"messages"`
 	TaskState *TaskState        `json:"taskState,omitempty"`
@@ -81,11 +83,18 @@ func (ts *TaskState) render() string {
 	return sb.String()
 }
 
-// SaveTo writes the full message history to path as JSON.
+// saveTo writes the full message history to path as JSON.
 func (s *Session) saveTo(path, id, agentName, cwd string) error {
+	return s.saveToFull(path, id, agentName, "", "", cwd)
+}
+
+// saveToFull writes session state including backend/model info.
+func (s *Session) saveToFull(path, id, agentName, backendName, modelName, cwd string) error {
 	ps := PersistedSession{
 		ID:        id,
 		Agent:     agentName,
+		Backend:   backendName,
+		Model:     modelName,
 		CWD:       cwd,
 		Messages:  s.messages,
 		TaskState: s.TaskState,
@@ -97,6 +106,19 @@ func (s *Session) saveTo(path, id, agentName, cwd string) error {
 	return os.WriteFile(path, data, 0600)
 }
 
+// LoadPersistedSession reads a PersistedSession from a JSON file.
+func LoadPersistedSession(path string) (*PersistedSession, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	var ps PersistedSession
+	if err := json.Unmarshal(data, &ps); err != nil {
+		return nil, fmt.Errorf("parse session %s: %w", path, err)
+	}
+	return &ps, nil
+}
+
 // RestoreSession reconstructs a Session from a persisted message list.
 func RestoreSession(messages []backend.Message) *Session {
 	s := &Session{messages: messages}

pkg/agent/types.go

@@ -139,6 +139,10 @@ type Core interface {
 	//   [ $(($(cat tcct) % 10)) -eq 0 ] && ...
 	ToolCallCount() int64
 
+	// SaveSession writes the current session state to the given path.
+	// The file includes all messages, agent/backend/model metadata, and task state.
+	SaveSession(path string) error
+
 	// Close releases resources associated with the session, including its
 	// temporary directory under /tmp/ollie/.
 	Close()