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/agent/agent.service.ts

@@ -1,4 +1,4 @@
-import { Inject, Injectable, Logger, type OnModuleDestroy } from '@nestjs/common';
+import { Inject, Injectable, Logger, Optional, type OnModuleDestroy } from '@nestjs/common';
 import {
   createAgentSession,
   DefaultResourceLoader,
@@ -24,6 +24,8 @@ import { createGitTools } from './tools/git-tools.js';
 import { createShellTools } from './tools/shell-tools.js';
 import { createWebTools } from './tools/web-tools.js';
 import type { SessionInfoDto } from './session.dto.js';
+import { SystemOverrideService } from '../preferences/system-override.service.js';
+import { PreferencesService } from '../preferences/preferences.service.js';
 
 export interface AgentSessionOptions {
   provider?: string;
@@ -55,6 +57,8 @@ export interface AgentSessionOptions {
    * take precedence over config values.
    */
   agentConfigId?: string;
+  /** ID of the user who owns this session. Used for preferences and system override lookups. */
+  userId?: string;
 }
 
 export interface AgentSession {
@@ -73,6 +77,8 @@ export interface AgentSession {
   sandboxDir: string;
   /** Tool names available in this session, or null when all tools are available. */
   allowedTools: string[] | null;
+  /** User ID that owns this session, used for preference lookups. */
+  userId?: string;
 }
 
 @Injectable()
@@ -89,6 +95,12 @@ export class AgentService implements OnModuleDestroy {
     @Inject(CoordService) private readonly coordService: CoordService,
     @Inject(McpClientService) private readonly mcpClientService: McpClientService,
     @Inject(SkillLoaderService) private readonly skillLoaderService: SkillLoaderService,
+    @Optional()
+    @Inject(SystemOverrideService)
+    private readonly systemOverride: SystemOverrideService | null,
+    @Optional()
+    @Inject(PreferencesService)
+    private readonly preferencesService: PreferencesService | null,
   ) {}
 
   /**
@@ -285,6 +297,7 @@ export class AgentService implements OnModuleDestroy {
       skillPromptAdditions: promptAdditions,
       sandboxDir,
       allowedTools,
+      userId: mergedOptions?.userId,
     };
 
     this.sessions.set(sessionId, session);
@@ -368,8 +381,20 @@ export class AgentService implements OnModuleDestroy {
       throw new Error(`No agent session found: ${sessionId}`);
     }
     session.promptCount += 1;
+
+    // Prepend session-scoped system override if present (renew TTL on each turn)
+    let effectiveMessage = message;
+    if (this.systemOverride) {
+      const override = await this.systemOverride.get(sessionId);
+      if (override) {
+        effectiveMessage = `[System Override]\n${override}\n\n${message}`;
+        await this.systemOverride.renew(sessionId);
+        this.logger.debug(`Applied system override for session ${sessionId}`);
+      }
+    }
+
     try {
-      await session.piSession.prompt(message);
+      await session.piSession.prompt(effectiveMessage);
     } catch (err) {
       this.logger.error(
         `Prompt failed for session=${sessionId}, messageLength=${message.length}`,