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/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();