refactor(FN-2134): remove legacy memory path exports and bootstrap aliases

- Remove legacy memory path constants/helpers from core exports and backend contract surface
- Simplify memory bootstrap to initialize canonical layered memory files without seeding from .fusion/memory.md
- Update core memory tests to assert canonical .fusion/memory/MEMORY.md behavior and legacy-path rejection
- Refresh dashboard and docs naming/references to reflect canonical long-term memory path semantics

packages/core/src/index.ts

@@ -415,8 +415,6 @@ export type {
 } from "./memory-insights.js";
 
 export {
-  MEMORY_FILE_PATH,
-  memoryFilePath,
   getDefaultMemoryScaffold,
   ensureMemoryFile,
   ensureMemoryFileWithBackend,
@@ -440,7 +438,6 @@ export {
   MEMORY_WORKSPACE_PATH,
   MEMORY_LONG_TERM_FILENAME,
   MEMORY_DREAMS_FILENAME,
-  LEGACY_MEMORY_FILE_PATH,
   QMD_INSTALL_COMMAND,
   QMD_REFRESH_INTERVAL_MS,
   memoryWorkspacePath,