fix: honor agent variant overrides (#1394)

* fix(shared): honor agent variant overrides

* test(shared): use model in fallback chain to verify override precedence

Address PR review: test now uses claude-opus-4-5 (which has default
variant 'max' in sisyphus chain) to properly verify that agent override
'high' takes precedence over the fallback chain's default variant.

src/shared/agent-variant.test.ts

@@ -83,6 +83,23 @@ describe("applyAgentVariant", () => {
 })
 
 describe("resolveVariantForModel", () => {
+  test("returns agent override variant when configured", () => {
+    // given - use a model in sisyphus chain (claude-opus-4-5 has default variant "max")
+    // to verify override takes precedence over fallback chain
+    const config = {
+      agents: {
+        sisyphus: { variant: "high" },
+      },
+    } as OhMyOpenCodeConfig
+    const model = { providerID: "anthropic", modelID: "claude-opus-4-5" }
+
+    // when
+    const variant = resolveVariantForModel(config, "sisyphus", model)
+
+    // then
+    expect(variant).toBe("high")
+  })
+
   test("returns correct variant for anthropic provider", () => {
     // given
     const config = {} as OhMyOpenCodeConfig