sessionfile: defer backend/model overrides until after agent= in handleCfg

Ensures manual model/backend settings always win over agent.json defaults
regardless of line order in ctl writes.

store/sessionfile.go

@@ -328,6 +328,7 @@ func (h *sessionHelper) cfgContent() string {
 func (h *sessionHelper) handleCfg(input string) error {
 	p := h.sess.Core.GenerationParams()
 	hasParams := false
+	var deferredBackend, deferredModel string
 	for _, line := range strings.Split(input, "\n") {
 		line = strings.TrimSpace(line)
 		if line == "" {
@@ -339,14 +340,28 @@ func (h *sessionHelper) handleCfg(input string) error {
 		}
 		k, v = strings.TrimSpace(k), strings.TrimSpace(v)
 		switch k {
-		case "backend", "model", "agent":
+		case "agent":
 			if v == "" {
 				continue
 			}
 			if h.sess.Core.IsRunning() {
 				return fmt.Errorf("cannot switch %s while agent is running", k)
 			}
-			h.sess.Core.Submit(h.sess.Ctx, "/"+k+" "+v, h.makePublish())
+			h.sess.Core.Submit(h.sess.Ctx, "/agent "+v, h.makePublish())
+		case "backend", "model":
+			if v == "" {
+				continue
+			}
+			if h.sess.Core.IsRunning() {
+				return fmt.Errorf("cannot switch %s while agent is running", k)
+			}
+			// Defer backend/model until after the loop so they override
+			// any values set by agent=.
+			if k == "backend" {
+				deferredBackend = v
+			} else {
+				deferredModel = v
+			}
 		case "cwd":
 			if v == "" {
 				continue
@@ -458,6 +473,13 @@ func (h *sessionHelper) handleCfg(input string) error {
 		// state → read-only, silently ignored
 		}
 	}
+	// Apply backend/model after agent= so manual overrides win.
+	if deferredBackend != "" {
+		h.sess.Core.Submit(h.sess.Ctx, "/backend "+deferredBackend, h.makePublish())
+	}
+	if deferredModel != "" {
+		h.sess.Core.Submit(h.sess.Ctx, "/model "+deferredModel, h.makePublish())
+	}
 	if hasParams {
 		if h.sess.Core.IsRunning() {
 			return fmt.Errorf("cannot change params while agent is running")