feat(FN-1420): add pluggable memory backend system

- Add FileMemoryBackend with atomic writes, persistence, and conflict resolution
- Add ReadOnlyMemoryBackend for read-only/external memory management
- Add memoryBackendType setting to select backend type (file or readonly)
- Add GET /api/memory/backend endpoint to query current backend status and capabilities
- Update AGENTS.md, README.md, and docs with architecture and settings guidance
- Add memory-backend.test.ts with comprehensive tests for all backends
- Add routes.test.ts coverage for /api/memory/backend endpoint
- Fix settings parity test to include new memoryBackendType key

packages/core/src/__tests__/settings-parity.test.ts

@@ -87,6 +87,7 @@ const PROJECT_KEYS: (keyof ProjectSettings)[] = [
   "insightExtractionSchedule",
   "insightExtractionMinIntervalMs",
   "memoryEnabled",
+  "memoryBackendType",
   "tokenCap",
   "runStepsInNewSessions",
   "maxParallelSteps",