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/merger.ts

@@ -1,4 +1,7 @@
-import { execSync } from "node:child_process";
+import { execSync, exec } from "node:child_process";
+import { promisify } from "node:util";
+
+const execAsync = promisify(exec);
 import { existsSync } from "node:fs";
 import { join } from "node:path";
 import { getTaskMergeBlocker, type TaskStore, type MergeResult, type MergeDetails, type WorkflowStep, type WorkflowStepResult, type Settings, type AgentPromptsConfig } from "@fusion/core";
@@ -325,15 +328,16 @@ async function runVerificationCommand(
   };
 
   try {
-    // Execute the command with timeout
-    const output = execSync(command, {
+    // Execute the command with timeout (non-blocking: uses async exec so the
+    // engine event loop keeps running while the child process executes)
+    const { stdout, stderr } = await execAsync(command, {
       cwd: rootDir,
       encoding: "utf-8",
       maxBuffer: VERIFICATION_COMMAND_MAX_BUFFER,
       timeout: 300_000, // 5 minute timeout for verification commands
-      stdio: ["pipe", "pipe", "pipe"],
     });
-    result.stdout = output;
+    result.stdout = stdout;
+    result.stderr = stderr;
     result.exitCode = 0;
     result.success = true;
     mergerLog.log(`${taskId}: ${type} command succeeded`);