fix(engine): run user commands via async exec to avoid blocking event loop

The merger's test/build verification, executor's worktreeInitCommand,
setupScript, and script-mode workflow steps all used execSync, which
blocks Node's event loop until the child process exits. A hanging
pnpm test could freeze the entire engine (no logs, heartbeats, or
other task progress) for the full 5-minute timeout.

Switch these call sites to promisify(exec) with awaited calls so the
engine keeps running while user-configured commands execute. Short
internal git plumbing (rev-parse, branch -d, worktree remove) still
uses execSync since those commands are bounded and measured in ms.

Document the rule in AGENTS.md under a new "Engine process rules"
section so future agents don't reintroduce blocking behavior.

Tests: update child_process mocks in merger.test, executor.test, and
restart.integration.test to route the new async exec through the
existing execSync mock and expose promisify.custom so destructuring
{ stdout, stderr } matches real child_process.exec semantics.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/engine/src/restart.integration.test.ts

@@ -30,6 +30,10 @@ vi.mock("./reviewer.js", () => ({
 }));
 vi.mock("node:child_process", () => ({
   execSync: vi.fn().mockReturnValue(Buffer.from("")),
+  exec: vi.fn((_cmd: any, opts: any, cb: any) => {
+    const callback = typeof opts === "function" ? opts : cb;
+    if (typeof callback === "function") callback(null, "", "");
+  }),
 }));
 vi.mock("node:fs", () => ({
   existsSync: vi.fn().mockReturnValue(true),