fs: remove dead switch cases from writeAgentCtl and writeAgentCfg

Remove no-op cases that did nothing but carry stale comments:
- ctl: backend (selection via cfg instead)
- ctl: skills (handled elsewhere)
- cfg: backend, model

fs/handlers.go

@@ -610,8 +610,6 @@ func writeAgentCfg(ctx HandlerCtx, data []byte) error {
 		if strings.TrimSpace(parts[1]) != "" {
 			ctx.Agent.SetName(strings.TrimSpace(parts[1]))
 		}
-	case "backend":
-	case "model":
 	default:
 		return fmt.Errorf("unknown cfg key: %s", parts[0])
 	}
@@ -639,8 +637,6 @@ func writeAgentCtl(ctx HandlerCtx, data []byte) error {
 		ctx.Agent.HandleCommand(ctx.Session.Ctx(), "/compact")
 	case "clear":
 		ctx.Agent.HandleCommand(ctx.Session.Ctx(), "/clear")
-	case "backend":
-		// Backend selection via cfg
 	case "model":
 		if len(cmd) > 1 {
 			if be := ctx.Agent.Backend(); be != nil {
@@ -655,8 +651,6 @@ func writeAgentCtl(ctx HandlerCtx, data []byte) error {
 		if len(cmd) > 1 {
 			ctx.Agent.SetCWD(strings.Join(cmd[1:], " "))
 		}
-	case "skills":
-		// skill list handled elsewhere
 	default:
 		return fmt.Errorf("unknown agent ctl command: %s", cmd[0])
 	}