feat: prioritize claude-opus-4-6 over claude-opus-4-5 in anthropic fallback chains

Add claude-opus-4-6 as the first anthropic provider entry before
claude-opus-4-5 across all agent and category fallback chains.
Also add high variant mapping for think-mode switcher.

src/agents/utils.test.ts

@@ -91,7 +91,7 @@ describe("createBuiltinAgents with model overrides", () => {
 
       // #then
       expect(agents.sisyphus).toBeDefined()
-      expect(agents.sisyphus.model).toBe("anthropic/claude-opus-4-5")
+      expect(agents.sisyphus.model).toBe("anthropic/claude-opus-4-6")
     } finally {
       cacheSpy.mockRestore()
       fetchSpy.mockRestore()
@@ -345,7 +345,7 @@ describe("createBuiltinAgents with requiresAnyModel gating (sisyphus)", () => {
 
       // #then
       expect(agents.sisyphus).toBeDefined()
-      expect(agents.sisyphus.model).toBe("anthropic/claude-opus-4-5")
+      expect(agents.sisyphus.model).toBe("anthropic/claude-opus-4-6")
     } finally {
       cacheSpy.mockRestore()
       fetchSpy.mockRestore()