feat(FN-3892): honor assigned runtime model for merger sessions

- Add resolveMergerSessionModel helper to centralize merger model selection precedence
- Route merger session creation paths through the helper for commit, push/rebase, and autostash recovery flows
- Add comprehensive engine tests for merger model resolution and fallback behavior
- Document merger model hierarchy in settings reference and add a patch changeset for @runfusion/fusion

Fusion-Task-Id: FN-3892

packages/engine/src/__tests__/agent-session-helpers.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { extractRuntimeHint } from "../agent-session-helpers.js";
+import { extractRuntimeHint, resolveMergerSessionModel } from "../agent-session-helpers.js";
 
 describe("extractRuntimeHint", () => {
   it("returns undefined for undefined config", () => {
@@ -22,3 +22,92 @@ describe("extractRuntimeHint", () => {
     expect(extractRuntimeHint({ runtimeHint: 42 })).toBeUndefined();
   });
 });
+
+describe("resolveMergerSessionModel", () => {
+  it("uses assigned agent runtime model when both provider and modelId are present", () => {
+    expect(
+      resolveMergerSessionModel(
+        {
+          defaultProviderOverride: "openai",
+          defaultModelIdOverride: "gpt-4.1",
+          defaultProvider: "anthropic",
+          defaultModelId: "claude-3-5-sonnet",
+        },
+        { model: "  anthropic/claude-3-5-sonnet-20241022  " },
+      ),
+    ).toEqual({
+      provider: "anthropic",
+      modelId: "claude-3-5-sonnet-20241022",
+    });
+  });
+
+  it("falls back to default override pair when runtime model is not fully specified", () => {
+    expect(
+      resolveMergerSessionModel(
+        {
+          defaultProviderOverride: "openai",
+          defaultModelIdOverride: "gpt-4.1",
+          defaultProvider: "anthropic",
+          defaultModelId: "claude-3-5-sonnet",
+        },
+        { modelProvider: "anthropic" },
+      ),
+    ).toEqual({
+      provider: "openai",
+      modelId: "gpt-4.1",
+    });
+  });
+
+  it("falls back to global defaults when no override pair is configured", () => {
+    expect(
+      resolveMergerSessionModel(
+        {
+          defaultProvider: "anthropic",
+          defaultModelId: "claude-3-5-sonnet",
+        },
+        { modelId: "claude-3-opus" },
+      ),
+    ).toEqual({
+      provider: "anthropic",
+      modelId: "claude-3-5-sonnet",
+    });
+  });
+
+  it("ignores partial override pairs and falls back to global defaults", () => {
+    expect(
+      resolveMergerSessionModel({
+        defaultProviderOverride: "openai",
+        defaultProvider: "anthropic",
+        defaultModelId: "claude-3-5-sonnet",
+      }),
+    ).toEqual({
+      provider: "anthropic",
+      modelId: "claude-3-5-sonnet",
+    });
+
+    expect(
+      resolveMergerSessionModel({
+        defaultModelIdOverride: "gpt-4.1",
+        defaultProvider: "anthropic",
+        defaultModelId: "claude-3-5-sonnet",
+      }),
+    ).toEqual({
+      provider: "anthropic",
+      modelId: "claude-3-5-sonnet",
+    });
+  });
+
+  it("works when assignedAgentRuntimeConfig is undefined", () => {
+    expect(
+      resolveMergerSessionModel({
+        defaultProviderOverride: "openai",
+        defaultModelIdOverride: "gpt-4.1",
+        defaultProvider: "anthropic",
+        defaultModelId: "claude-3-5-sonnet",
+      }),
+    ).toEqual({
+      provider: "openai",
+      modelId: "gpt-4.1",
+    });
+  });
+});