fix(FN-2078): adopt canonical .fusion/memory paths

- Update core and engine prompt text to reference .fusion/memory/ with MEMORY.md and daily-note guidance
- Allow worktree tool boundary access to .fusion/memory/ files while preserving legacy .fusion/memory.md compatibility
- Revise memory compaction/insight extraction messaging and related type/docs comments to use canonical memory paths
- Refresh dashboard and test expectations across core/engine/dashboard/docs for the new memory path wording

packages/dashboard/app/components/MemoryView.tsx

@@ -95,7 +95,7 @@ function countTotalInsights(categories: ParsedInsightCategory[]): number {
 function getBackendDisplayName(backend: string): string {
   switch (backend) {
     case "file":
-      return "File (.fusion/memory.md)";
+      return "File (.fusion/memory/MEMORY.md)";
     case "readonly":
       return "Read-Only";
     case "qmd":