feat(gateway): PreferencesService + /preferences REST + /system Valkey override (P8-011)

- PreferencesService: platform defaults, user overrides, IMMUTABLE_KEYS enforcement
- PreferencesController: GET /api/preferences, POST /api/preferences, DELETE /api/preferences/:key
- PreferencesModule: global module exporting PreferencesService and SystemOverrideService
- SystemOverrideService: Valkey-backed session-scoped system prompt override with 5-min TTL + renew
- CommandRegistryService: register /system command (socket execution)
- CommandExecutorService: handle /system command via SystemOverrideService
- AgentService: inject system override before each prompt turn, renew TTL; store userId in session
- ChatGateway: pass userId when creating agent sessions
- PreferencesService unit tests: 11 tests covering defaults, overrides, enforcement wins, immutable key errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/commands/command-registry.service.ts

@@ -156,6 +156,22 @@ export class CommandRegistryService implements OnModuleInit {
         execution: 'rest',
         available: true,
       },
+      {
+        name: 'system',
+        description: 'Set session-scoped system prompt override',
+        aliases: [],
+        args: [
+          {
+            name: 'override',
+            type: 'string',
+            optional: false,
+            description: 'System prompt text to inject for this session',
+          },
+        ],
+        scope: 'core',
+        execution: 'socket',
+        available: true,
+      },
       {
         name: 'status',
         description: 'Show session and connection status',