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/engine/src/__tests__/merger-commit-strategy.real-git.test.ts

@@ -0,0 +1,141 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { execSync, spawnSync } from "node:child_process";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import type { Settings, Task, TaskStore } from "@fusion/core";
+
+type TaskWithPromptOverride = Partial<Task> & Pick<Task, "id"> & { prompt?: string };
+import { DEFAULT_SETTINGS } from "@fusion/core";
+import { aiMergeTask } from "../merger.js";
+
+const hasGit = spawnSync("git", ["--version"], { stdio: "pipe" }).status === 0;
+const describeIfGit = hasGit ? describe : describe.skip;
+
+function git(repo: string, command: string): string {
+  return execSync(command, { cwd: repo, encoding: "utf-8", stdio: ["pipe", "pipe", "pipe"] }).trim();
+}
+
+function makeTask(overrides: TaskWithPromptOverride): Task {
+  const { id, ...rest } = overrides;
+  return {
+    ...rest,
+    id,
+    title: overrides.title ?? id,
+    description: overrides.description ?? id,
+    column: overrides.column ?? "in-review",
+    dependencies: overrides.dependencies ?? [],
+    steps: overrides.steps ?? [],
+    currentStep: overrides.currentStep ?? 0,
+    log: overrides.log ?? [],
+    createdAt: overrides.createdAt ?? new Date().toISOString(),
+    updatedAt: overrides.updatedAt ?? new Date().toISOString(),
+  } as Task;
+}
+
+function createStore(task: Task, settings: Partial<Settings>): TaskStore {
+  let currentTask = { ...task };
+  const mergedSettings: Settings = {
+    ...DEFAULT_SETTINGS,
+    mergeStrategy: "direct",
+    directMergeCommitStrategy: "auto",
+    autoMerge: true,
+    includeTaskIdInCommit: false,
+    commitAuthorEnabled: false,
+    useAiMergeCommitSummary: false,
+    ...settings,
+  } as Settings;
+
+  return {
+    getTask: vi.fn(async () => currentTask),
+    getSettings: vi.fn(async () => mergedSettings),
+    listTasks: vi.fn(async () => [currentTask]),
+    updateTask: vi.fn(async (_id: string, updates: Partial<Task>) => {
+      currentTask = { ...currentTask, ...updates, updatedAt: new Date().toISOString() } as Task;
+      return currentTask;
+    }),
+    moveTask: vi.fn(async (_id: string, column: Task["column"]) => {
+      currentTask = {
+        ...currentTask,
+        column,
+        columnMovedAt: new Date().toISOString(),
+        updatedAt: new Date().toISOString(),
+      } as Task;
+      return currentTask;
+    }),
+    logEntry: vi.fn(async () => undefined),
+    appendAgentLog: vi.fn(async () => undefined),
+    updateSettings: vi.fn(async () => mergedSettings),
+    getActiveMergingTask: vi.fn(() => null),
+    emit: vi.fn(),
+    on: vi.fn(),
+    clearStaleExecutionStartBranchReferences: vi.fn(() => []),
+    getVerificationCacheHit: vi.fn(() => null),
+    recordVerificationCachePass: vi.fn(() => undefined),
+    upsertTaskCommitAssociation: vi.fn(async () => undefined),
+  } as unknown as TaskStore;
+}
+
+describeIfGit("aiMergeTask direct merge commit routing (real git)", () => {
+  const repos: string[] = [];
+
+  afterEach(() => {
+    for (const repo of repos.splice(0)) {
+      rmSync(repo, { recursive: true, force: true });
+    }
+  });
+
+  function setupRepo(): { repo: string; initSha: string } {
+    const repo = mkdtempSync(join(tmpdir(), "fusion-merger-commit-strategy-"));
+    repos.push(repo);
+    git(repo, "git init -b main");
+    git(repo, 'git config user.email "test@example.com"');
+    git(repo, 'git config user.name "Test User"');
+    writeFileSync(join(repo, "README.md"), "init\n", "utf-8");
+    git(repo, "git add README.md && git commit -m 'chore: init'");
+    return { repo, initSha: git(repo, "git rev-parse HEAD") };
+  }
+
+  it("auto-routes multi-substantive branches to history-preserving direct merge", async () => {
+    const { repo, initSha } = setupRepo();
+    const branch = "fusion/fn-4069-test";
+
+    git(repo, `git checkout -b ${branch}`);
+    writeFileSync(join(repo, "src-fix.ts"), "export const fix = 1;\n", "utf-8");
+    git(repo, "git add src-fix.ts && git commit -m 'fix: preserve original bugfix'");
+
+    writeFileSync(join(repo, ".changeset-fn-4069.md"), "noop\n", "utf-8");
+    git(repo, "mkdir -p .changeset && mv .changeset-fn-4069.md .changeset/fn-4069.md && git add .changeset/fn-4069.md && git commit -m 'chore: add changeset'");
+
+    writeFileSync(join(repo, "src-style.css"), ".root { display: block; }\n", "utf-8");
+    git(repo, "git add src-style.css && git commit -m 'feat: preserve follow-up polish'");
+    git(repo, "git checkout main");
+
+    const task = makeTask({
+      id: "FN-4069",
+      branch,
+      baseBranch: "main",
+      column: "in-review",
+      prompt: "# Task\n",
+    });
+    const store = createStore(task, {});
+
+    await aiMergeTask(store, repo, "FN-4069");
+
+    expect((store.moveTask as ReturnType<typeof vi.fn>).mock.calls.some(([, column]) => column === "done")).toBe(true);
+
+    const subjects = git(repo, `git log --reverse --format=%s ${initSha}..HEAD`).split("\n");
+    expect(subjects).toEqual([
+      "fix: preserve original bugfix",
+      "chore: add changeset",
+      "feat: preserve follow-up polish",
+    ]);
+
+    const landedShas = git(repo, `git rev-list --reverse ${initSha}..HEAD`).split("\n");
+    expect(landedShas).toHaveLength(3);
+    for (const sha of landedShas) {
+      const body = git(repo, `git log -1 --format=%B ${sha}`);
+      expect(body).toContain("Fusion-Task-Id: FN-4069");
+    }
+  }, 20_000);
+});