fix(FN-834): fix branch prefix drift, add merger branch guard, and fix test OOM

- Fix resolveBaseBranch to use stored branch name and consistent fusion/ prefix
  for both explicit deps and blockedBy paths (was using kb/ for blockedBy)
- Add main branch checkout verification in merger before squash merge to prevent
  feature code from landing on wrong branch lineage
- Align all branch prefix references from stale kb/ to fusion/ across executor,
  merger, store, and routes
- Fix executor test OOM by mocking merger fully, adding fake timers to retry
  tests, and switching vitest pool to vmThreads
- Update all test assertions to use fusion/ branch prefix

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

packages/dashboard/app/hooks/useLiveTranscript.ts

@@ -0,0 +1,43 @@
+import { useState, useEffect, useRef } from "react";
+
+/** Log entry from an agent's execution stream */
+export interface TranscriptEntry {
+  type: string;
+  content: string;
+  timestamp?: string;
+}
+
+export function useLiveTranscript(taskId: string | undefined) {
+  const [entries, setEntries] = useState<TranscriptEntry[]>([]);
+  const [isConnected, setIsConnected] = useState(false);
+  const esRef = useRef<EventSource | null>(null);
+
+  useEffect(() => {
+    if (!taskId) {
+      setEntries([]);
+      setIsConnected(false);
+      return;
+    }
+
+    const es = new EventSource(`/api/tasks/${encodeURIComponent(taskId)}/logs/stream`);
+    esRef.current = es;
+
+    es.addEventListener("agent:log", (event) => {
+      try {
+        const entry = JSON.parse(event.data) as TranscriptEntry;
+        setEntries(prev => [entry, ...prev]);
+      } catch { /* skip */ }
+    });
+
+    es.addEventListener("open", () => setIsConnected(true));
+    es.addEventListener("error", () => setIsConnected(false));
+
+    return () => {
+      es.close();
+      esRef.current = null;
+      setIsConnected(false);
+    };
+  }, [taskId]);
+
+  return { entries, isConnected };
+}