feat(FN-5089): complete Step 1 — add commit-msg hook installer

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

packages/engine/src/worktree-hooks.ts

@@ -6,6 +6,7 @@ import { promisify } from "node:util";
 const execAsync = promisify(exec);
 
 export const DEFAULT_ALLOWED_BRANCH_PATTERNS = ["^fusion/step-\\d+-[a-z0-9-]+$"] as const;
+const COMMIT_MSG_HOOK_MARKER = "# fusion-managed-commit-msg-hook";
 
 function toShellCasePattern(pattern: string): string {
   return pattern
@@ -61,6 +62,42 @@ async function resolveGitPath(worktreePath: string, gitPath: string): Promise<st
   }
 }
 
+export function buildCommitMsgTrailerHook(
+  taskId: string,
+  options: {
+    taskPrefix?: string;
+    trailerName?: string;
+  } = {}
+): string {
+  const taskPrefix = (options.taskPrefix ?? "FN").trim() || "FN";
+  const trailerName = (options.trailerName ?? "Fusion-Task-Id").trim() || "Fusion-Task-Id";
+
+  return `#!/bin/sh
+set -eu
+${COMMIT_MSG_HOOK_MARKER}
+# fusion-task-id-seed: ${taskId}
+
+TASK_FILE=$(git rev-parse --git-path fusion-task-id)
+[ -f "$TASK_FILE" ] || exit 0
+TASK_ID=$(cat "$TASK_FILE")
+[ -n "$TASK_ID" ] || exit 0
+
+PREFIX=${JSON.stringify(taskPrefix)}
+case "$TASK_ID" in
+  ${taskPrefix}-*) ;;
+  *) TASK_ID="$PREFIX-$(printf '%s' "$TASK_ID" | sed -E "s/^${taskPrefix}-//i")" ;;
+esac
+
+TRAILER_NAME=${JSON.stringify(trailerName)}
+
+git interpret-trailers \
+  --in-place \
+  --if-exists doNothing \
+  --trailer "$TRAILER_NAME: $TASK_ID" \
+  "$1"
+`;
+}
+
 async function writeFileAtomic(targetPath: string, content: string, mode?: number): Promise<void> {
   await execAsync(`mkdir -p ${JSON.stringify(dirname(targetPath))}`);
   const tmpPath = `${targetPath}.tmp`;
@@ -71,10 +108,35 @@ async function writeFileAtomic(targetPath: string, content: string, mode?: numbe
   await fs.rename(tmpPath, targetPath);
 }
 
+async function installCommitMsgHook(input: {
+  worktreePath: string;
+  taskId: string;
+  taskPrefix: string;
+  trailerName: string;
+}): Promise<void> {
+  const hookPath = await resolveGitPath(input.worktreePath, "hooks/commit-msg");
+  const existing = await fs.readFile(hookPath, "utf-8").catch(() => null);
+  if (existing && !existing.includes(COMMIT_MSG_HOOK_MARKER)) {
+    console.warn(
+      `[worktree-hooks] commit-msg hook already exists at ${hookPath}; skipping Fusion trailer hook install for ${input.taskId}`
+    );
+    return;
+  }
+
+  const hook = buildCommitMsgTrailerHook(input.taskId, {
+    taskPrefix: input.taskPrefix,
+    trailerName: input.trailerName,
+  });
+  await writeFileAtomic(hookPath, hook, 0o755);
+}
+
 export async function installTaskWorktreeIdentityGuard(input: {
   worktreePath: string;
   taskId: string;
   allowedBranchPatterns?: readonly string[];
+  commitMsgHookEnabled?: boolean;
+  taskPrefix?: string;
+  taskAttributionTrailerName?: string;
 }): Promise<void> {
   const hook = buildIdentityGuardHook(input.taskId, input.allowedBranchPatterns ?? DEFAULT_ALLOWED_BRANCH_PATTERNS);
   const metadataPath = await resolveGitPath(input.worktreePath, "fusion-task-id");
@@ -82,4 +144,13 @@ export async function installTaskWorktreeIdentityGuard(input: {
 
   await writeFileAtomic(metadataPath, `${input.taskId}\n`);
   await writeFileAtomic(hookPath, hook, 0o755);
+
+  if (input.commitMsgHookEnabled !== false) {
+    await installCommitMsgHook({
+      worktreePath: input.worktreePath,
+      taskId: input.taskId,
+      taskPrefix: input.taskPrefix ?? "FN",
+      trailerName: input.taskAttributionTrailerName ?? "Fusion-Task-Id",
+    });
+  }
 }