feat(FN-2133): standardize project memory path guidance to .fusion/memory/

- Update executor, reviewer, core prompt templates, and path-boundary messaging to reference the .fusion/memory/ directory instead of a single MEMORY.md file
- Broaden worktree boundary checks in pi path validation to allow .fusion/memory/ directory access from task worktrees
- Align memory backend metadata and dashboard backend labels to display file backend storage as .fusion/memory/
- Refresh core/engine tests to assert the new directory-based memory wording and boundary behavior

packages/dashboard/app/components/MemoryView.tsx

@@ -93,7 +93,7 @@ function countTotalInsights(categories: ParsedInsightCategory[]): number {
 function getBackendDisplayName(backend: string): string {
   switch (backend) {
     case "file":
-      return "File (.fusion/memory/MEMORY.md)";
+      return "File (.fusion/memory/)";
     case "readonly":
       return "Read-Only";
     case "qmd":