session: add Checkpoint for typed subflow handoff

pkg/agent/session.go

@@ -133,6 +133,22 @@ func newSession(goal string) *Session {
 	return &Session{goal: goal}
 }
 
+// Checkpoint forks the session: returns a new Session that inherits the given
+// TaskState but starts with a clean message history. This enables narrow-context
+// sub-agents that know what to do without inheriting all parent message noise.
+func (s *Session) Checkpoint(ts TaskState) *Session {
+	child := &Session{
+		goal:      ts.Objective,
+		TaskState: &ts,
+	}
+	// Seed with a user message so the child has a valid initial turn.
+	child.messages = []backend.Message{{
+		Role:    "user",
+		Content: ts.render(),
+	}}
+	return child
+}
+
 func (s *Session) history() []backend.Message {
 	return s.messages
 }