test(FN-5089): add commit-msg hook coverage and wiring

Fusion-Task-Id: FN-5089
Fusion-Task-Lineage: ad7ea32a-c024-45bb-b68e-b65665b9b7c6

packages/engine/src/__tests__/real-git/commit-msg-trailer.real-git.test.ts

@@ -0,0 +1,55 @@
+import { describe, it, expect } from "vitest";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { execSync } from "node:child_process";
+import { installTaskWorktreeIdentityGuard } from "../../worktree-hooks.js";
+
+function git(dir: string, cmd: string): string {
+  return execSync(cmd, { cwd: dir, stdio: "pipe" }).toString().trim();
+}
+
+describe("commit-msg trailer hook (real git)", () => {
+  it("appends and preserves Fusion-Task-Id trailer in fusion worktrees", async () => {
+    const rootDir = mkdtempSync(join(tmpdir(), "fn-5089-commit-msg-"));
+    const worktreeDir = join(rootDir, "wt-kb");
+
+    try {
+      git(rootDir, "git init -b main");
+      git(rootDir, 'git config user.email "test@example.com"');
+      git(rootDir, 'git config user.name "Test"');
+      writeFileSync(join(rootDir, "README.md"), "init\n");
+      git(rootDir, "git add README.md && git commit -m 'init'");
+
+      git(rootDir, "git worktree add -b fusion/kb-7 wt-kb HEAD");
+      await installTaskWorktreeIdentityGuard({
+        worktreePath: worktreeDir,
+        taskId: "KB-7",
+        taskPrefix: "KB",
+        taskAttributionTrailerName: "Fusion-Task-Id",
+      });
+
+      git(worktreeDir, "git commit --allow-empty -m 'feat(KB-7): first'");
+      const firstBody = git(worktreeDir, "git log -1 --format=%B");
+      expect(firstBody).toContain("Fusion-Task-Id: KB-7");
+      const firstTrailers = git(worktreeDir, "git log -1 --format=%B | git interpret-trailers --parse");
+      expect(firstTrailers).toContain("Fusion-Task-Id: KB-7");
+
+      git(worktreeDir, "git commit --amend --no-edit --allow-empty");
+      const amendNoEditBody = git(worktreeDir, "git log -1 --format=%B");
+      expect((amendNoEditBody.match(/Fusion-Task-Id:\s*KB-7/g) ?? []).length).toBe(1);
+
+      git(worktreeDir, "git commit --amend -m 'feat(KB-7): rewritten' --allow-empty");
+      const rewrittenBody = git(worktreeDir, "git log -1 --format=%B");
+      expect(rewrittenBody).toContain("feat(KB-7): rewritten");
+      expect((rewrittenBody.match(/Fusion-Task-Id:\s*KB-7/g) ?? []).length).toBe(1);
+
+      writeFileSync(join(rootDir, "outside.txt"), "outside\n");
+      git(rootDir, "git add outside.txt && git commit -m 'chore: root commit'");
+      const rootBody = git(rootDir, "git log -1 --format=%B");
+      expect(rootBody).not.toContain("Fusion-Task-Id:");
+    } finally {
+      rmSync(rootDir, { recursive: true, force: true });
+    }
+  }, 30_000);
+});