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

docs/settings-reference.md

@@ -121,6 +121,7 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
 | `insightExtractionSchedule` | `string` | `"0 2 * * *"` | Insight extraction cron schedule. |
 | `insightExtractionMinIntervalMs` | `number` | `86400000` | Minimum interval between insight extraction runs (24h). |
 | `memoryEnabled` | `boolean` | `true` | Enable project memory integration. |
+| `memoryBackendType` | `string` | `"file"` | Memory backend type: `file` or `readonly`. |
 | `runStepsInNewSessions` | `boolean` | `false` | Run each task step in a fresh agent session. |
 | `maxParallelSteps` | `number` | `2` | Max concurrent step sessions (1–4). |
 | `agentPrompts` | `object` | `undefined` | Custom agent prompt templates + role assignments. |