feat(FN-4069): add direct merge commit routing to merger

Adds direct merge commit routing to the merger, allowing non-squash merges to bypass the squash-audit path when configured. The feature includes new `mergeCommitStrategy` settings, updated dashboard UI, expanded merger lifecycle tests, and documentation.

Fusion-Task-Id: FN-4069

packages/core/src/__tests__/settings-parity.test.ts

@@ -77,6 +77,12 @@ describe("settings key parity", () => {
     expect(DEFAULT_PROJECT_SETTINGS.completionDocumentationMode).toBe("off");
   });
 
+  it("keeps directMergeCommitStrategy project-scoped with auto default", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.directMergeCommitStrategy).toBe("auto");
+    expect(isProjectSettingsKey("directMergeCommitStrategy")).toBe(true);
+    expect(isGlobalSettingsKey("directMergeCommitStrategy")).toBe(false);
+  });
+
   it("keeps task stuck timeout active by default without coupling to workflow step timeout", () => {
     expect(DEFAULT_PROJECT_SETTINGS.taskStuckTimeoutMs).toBe(600_000);
     expect(DEFAULT_PROJECT_SETTINGS.workflowStepTimeoutMs).toBe(360_000);

packages/core/src/__tests__/settings-validation.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
+  validateDirectMergeCommitStrategy,
   validateGithubAuthMode,
   validateGithubRepoSlug,
   validateUnavailableNodePolicy,
@@ -19,6 +20,20 @@ describe("settings-validation", () => {
     });
   });
 
+  describe("validateDirectMergeCommitStrategy", () => {
+    it("accepts supported direct-merge routing values", () => {
+      expect(validateDirectMergeCommitStrategy("auto")).toBe("auto");
+      expect(validateDirectMergeCommitStrategy("always-squash")).toBe("always-squash");
+      expect(validateDirectMergeCommitStrategy("always-rebase")).toBe("always-rebase");
+    });
+
+    it("returns undefined for invalid routing values", () => {
+      expect(validateDirectMergeCommitStrategy("squash")).toBeUndefined();
+      expect(validateDirectMergeCommitStrategy(123)).toBeUndefined();
+      expect(validateDirectMergeCommitStrategy(undefined)).toBeUndefined();
+    });
+  });
+
   describe("validateGithubAuthMode", () => {
     it("accepts supported auth modes", () => {
       expect(validateGithubAuthMode("gh-cli")).toBe("gh-cli");

packages/core/src/__tests__/store-settings.test.ts

@@ -61,6 +61,15 @@ describe("TaskStore", () => {
       const settings = await harness.store().getSettings();
       expect(settings.mergeStrategy).toBe("pull-request");
     });
+
+    it("defaults directMergeCommitStrategy to auto and persists updates", async () => {
+      const defaults = await harness.store().getSettings();
+      expect(defaults.directMergeCommitStrategy).toBe("auto");
+
+      await harness.store().updateSettings({ directMergeCommitStrategy: "always-rebase" });
+      const settings = await harness.store().getSettings();
+      expect(settings.directMergeCommitStrategy).toBe("always-rebase");
+    });
   });
 
   // ── Planning/Validator Model Settings ────────────────────────────