fix(engine): make global pause actually stop in-flight verification

Hitting Stop (globalPause) disposed the AI merge agent session but left
the spawned `pnpm test` / `pnpm build` child processes running until
they finished naturally. With recurring flaky-test loops at Step 5,
that meant Stop had no visible effect — new test runs kept piling up
across multiple worktrees.

Two gaps:
- project-engine.ts onGlobalPause never called mergeAbortController.abort(),
  so subsequent verification commands (gated by the signal) weren't cancelled.
- merger.ts execWithProcessGroup only listened to its own internal
  timeout — passing an AbortSignal had no effect on the in-flight
  child process group.

Fix: abort the controller on global pause, and have execWithProcessGroup
SIGTERM/SIGKILL the detached process group when its signal aborts.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/merger.ts

@@ -16,9 +16,17 @@ const execAsync = promisify(exec);
  */
 async function execWithProcessGroup(
   command: string,
-  options: { cwd: string; timeout: number; maxBuffer: number },
-): Promise<{ stdout: string; stderr: string; bufferOverflow: boolean }> {
+  options: { cwd: string; timeout: number; maxBuffer: number; signal?: AbortSignal },
+): Promise<{ stdout: string; stderr: string; bufferOverflow: boolean; aborted?: boolean }> {
   return new Promise((resolve, reject) => {
+    if (options.signal?.aborted) {
+      reject(Object.assign(
+        new Error(`Command aborted before start: ${command}`),
+        { code: "ABORT_ERR", aborted: true, stdout: "", stderr: "" },
+      ));
+      return;
+    }
+
     const child = spawn(command, {
       cwd: options.cwd,
       shell: true,
@@ -31,20 +39,34 @@ async function execWithProcessGroup(
     let stdoutOverflow = false;
     let stderrOverflow = false;
     let timedOut = false;
+    let aborted = false;
     let settled = false;
 
+    const killTree = (sig: NodeJS.Signals) => {
+      if (child.pid === undefined) return;
+      try { process.kill(-child.pid, sig); } catch { /* group may already be gone */ }
+    };
+
     const timer = setTimeout(() => {
       timedOut = true;
-      if (child.pid !== undefined) {
-        try { process.kill(-child.pid, "SIGTERM"); } catch { /* group may already be gone */ }
-        setTimeout(() => {
-          if (settled) return;
-          try { process.kill(-(child.pid as number), "SIGKILL"); } catch { /* ignore */ }
-        }, 5_000).unref();
-      }
+      killTree("SIGTERM");
+      setTimeout(() => {
+        if (settled) return;
+        killTree("SIGKILL");
+      }, 5_000).unref();
     }, options.timeout);
     timer.unref();
 
+    const onAbort = () => {
+      aborted = true;
+      killTree("SIGTERM");
+      setTimeout(() => {
+        if (settled) return;
+        killTree("SIGKILL");
+      }, 5_000).unref();
+    };
+    options.signal?.addEventListener("abort", onAbort, { once: true });
+
     child.stdout?.on("data", (chunk: Buffer) => {
       if (stdoutOverflow) return;
       if (stdout.length + chunk.length > options.maxBuffer) {
@@ -68,6 +90,14 @@ async function execWithProcessGroup(
       if (settled) return;
       settled = true;
       clearTimeout(timer);
+      options.signal?.removeEventListener("abort", onAbort);
+      if (aborted) {
+        reject(Object.assign(
+          new Error(`Command aborted: ${command}`),
+          { code: "ABORT_ERR", aborted: true, stdout, stderr, killed: true },
+        ));
+        return;
+      }
       if (timedOut) {
         reject(Object.assign(
           new Error(`Command timed out after ${options.timeout}ms: ${command}`),
@@ -707,6 +737,7 @@ async function runVerificationCommand(
       cwd: rootDir,
       timeout: VERIFICATION_COMMAND_TIMEOUT_MS,
       maxBuffer: VERIFICATION_COMMAND_MAX_BUFFER,
+      signal,
     });
 
     throwIfAborted(signal, taskId);

packages/engine/src/project-engine.ts

@@ -1327,9 +1327,17 @@ export class ProjectEngine {
   // ── Settings event listeners ──
 
   private wireSettingsListeners(store: TaskStore): void {
-    // 1. Global pause — terminate active merge session
+    // 1. Global pause — terminate active merge session AND abort any running
+    // deterministic verification (pnpm test/build). The abort controller gates
+    // both the AI merge agent and the spawned child processes; without it,
+    // verification commands keep churning until they finish naturally.
     const onGlobalPause = ({ settings, previous }: { settings: Settings; previous: Settings }) => {
       if (settings.globalPause && !previous.globalPause) {
+        if (this.mergeAbortController) {
+          runtimeLog.log("Global pause — aborting in-flight merge verification");
+          this.mergeAbortController.abort();
+          this.mergeAbortController = null;
+        }
         if (this.activeMergeSession) {
           runtimeLog.log("Global pause — terminating active merge session");
           this.activeMergeSession.dispose();