feat(model-routing): enable dynamic routing by default (#3120)

* feat(model-routing): enable dynamic routing by default

Change defaultRoutingConfig().enabled from false to true so that
dynamic model routing (tier-based downgrading for light/standard
tasks) is active out of the box. Users can still disable it via
dynamic_routing.enabled: false in PREFERENCES.md.

This is a behavioral change: sessions that previously used the
configured model for all tasks will now automatically downgrade
to cheaper models for light and standard complexity tasks.

* test(model-routing): verify dynamic routing enabled by default

Tests that defaultRoutingConfig returns enabled: true and all
routing features are active.

src/resources/extensions/gsd/model-router.ts

@@ -225,7 +225,7 @@ export function escalateTier(currentTier: ComplexityTier): ComplexityTier | null
  */
 export function defaultRoutingConfig(): DynamicRoutingConfig {
   return {
-    enabled: false,
+    enabled: true,
     escalate_on_failure: true,
     budget_pressure: true,
     cross_provider: true,

src/resources/extensions/gsd/tests/dynamic-routing-default.test.ts

@@ -0,0 +1,20 @@
+/**
+ * Dynamic routing default — verifies routing is enabled by default.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import { defaultRoutingConfig } from "../model-router.js";
+
+test("defaultRoutingConfig returns enabled: true", () => {
+  const config = defaultRoutingConfig();
+  assert.equal(config.enabled, true, "dynamic routing should be enabled by default");
+});
+
+test("defaultRoutingConfig enables all routing features", () => {
+  const config = defaultRoutingConfig();
+  assert.equal(config.escalate_on_failure, true);
+  assert.equal(config.budget_pressure, true);
+  assert.equal(config.cross_provider, true);
+  assert.equal(config.hooks, true);
+});